Author name code: vonderluehe ADS astronomy entries on 2022-09-14 author:"von der Luehe, Oskar" ------------------------------------------------------------------------ Title: The National Science Foundation's Daniel K. Inouye Solar Telescope — Status Update Authors: Rimmele, T.; Woeger, F.; Tritschler, A.; Casini, R.; de Wijn, A.; Fehlmann, A.; Harrington, D.; Jaeggli, S.; Anan, T.; Beck, C.; Cauzzi, G.; Schad, T.; Criscuoli, S.; Davey, A.; Lin, H.; Kuhn, J.; Rast, M.; Goode, P.; Knoelker, M.; Rosner, R.; von der Luehe, O.; Mathioudakis, M.; Dkist Team Bibcode: 2021AAS...23810601R Altcode: The National Science Foundation's 4m Daniel K. Inouye Solar Telescope (DKIST) on Haleakala, Maui is now the largest solar telescope in the world. DKIST's superb resolution and polarimetric sensitivity will enable astronomers to unravel many of the mysteries the Sun presents, including the origin of solar magnetism, the mechanisms of coronal heating and drivers of flares and coronal mass ejections. Five instruments, four of which provide highly sensitive measurements of solar magnetic fields, including the illusive magnetic field of the faint solar corona. The DKIST instruments will produce large and complex data sets, which will be distributed through the NSO/DKIST Data Center. DKIST has achieved first engineering solar light in December of 2019. Due to COVID the start of the operations commissioning phase is delayed and is now expected for fall of 2021. We present a status update for the construction effort and progress with the operations commissioning phase. Title: DKIST First-light Instrumentation Authors: Woeger, F.; Rimmele, T.; Casini, R.; von der Luehe, O.; Lin, H.; Kuhn, J.; Dkist Team Bibcode: 2021AAS...23810602W Altcode: The NSF's Daniel K. Inouye Solar Telescope's (DKIST) four meter aperture and state-of-the-art wavefront correction system and instrumentation will facilitate new insights into the complexities of the solar atmosphere. We will describe the details and status of the diverse first light instruments, including the high order adaptive optics system, that are being commissioned: The Visible Spectro-Polarimeter (ViSP), the Visible Broadband Imager (VBI), the Visible Tunable Filter (VTF), the Diffraction-Limited Spectro-Polarimeter (DL-NIRSP) and the Cryogenic Spectro-Polarimeter (Cryo-NIRSP). We will present first data demonstrating the telescope's instrument systems performance. Title: The Daniel K. Inouye Solar Telescope - Observatory Overview Authors: Rimmele, Thomas R.; Warner, Mark; Keil, Stephen L.; Goode, Philip R.; Knölker, Michael; Kuhn, Jeffrey R.; Rosner, Robert R.; McMullin, Joseph P.; Casini, Roberto; Lin, Haosheng; Wöger, Friedrich; von der Lühe, Oskar; Tritschler, Alexandra; Davey, Alisdair; de Wijn, Alfred; Elmore, David F.; Fehlmann, André; Harrington, David M.; Jaeggli, Sarah A.; Rast, Mark P.; Schad, Thomas A.; Schmidt, Wolfgang; Mathioudakis, Mihalis; Mickey, Donald L.; Anan, Tetsu; Beck, Christian; Marshall, Heather K.; Jeffers, Paul F.; Oschmann, Jacobus M.; Beard, Andrew; Berst, David C.; Cowan, Bruce A.; Craig, Simon C.; Cross, Eric; Cummings, Bryan K.; Donnelly, Colleen; de Vanssay, Jean-Benoit; Eigenbrot, Arthur D.; Ferayorni, Andrew; Foster, Christopher; Galapon, Chriselle Ann; Gedrites, Christopher; Gonzales, Kerry; Goodrich, Bret D.; Gregory, Brian S.; Guzman, Stephanie S.; Guzzo, Stephen; Hegwer, Steve; Hubbard, Robert P.; Hubbard, John R.; Johansson, Erik M.; Johnson, Luke C.; Liang, Chen; Liang, Mary; McQuillen, Isaac; Mayer, Christopher; Newman, Karl; Onodera, Brialyn; Phelps, LeEllen; Puentes, Myles M.; Richards, Christopher; Rimmele, Lukas M.; Sekulic, Predrag; Shimko, Stephan R.; Simison, Brett E.; Smith, Brett; Starman, Erik; Sueoka, Stacey R.; Summers, Richard T.; Szabo, Aimee; Szabo, Louis; Wampler, Stephen B.; Williams, Timothy R.; White, Charles Bibcode: 2020SoPh..295..172R Altcode: We present an overview of the National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST), its instruments, and support facilities. The 4 m aperture DKIST provides the highest-resolution observations of the Sun ever achieved. The large aperture of DKIST combined with state-of-the-art instrumentation provide the sensitivity to measure the vector magnetic field in the chromosphere and in the faint corona, i.e. for the first time with DKIST we will be able to measure and study the most important free-energy source in the outer solar atmosphere - the coronal magnetic field. Over its operational lifetime DKIST will advance our knowledge of fundamental astronomical processes, including highly dynamic solar eruptions that are at the source of space-weather events that impact our technological society. Design and construction of DKIST took over two decades. DKIST implements a fast (f/2), off-axis Gregorian optical design. The maximum available field-of-view is 5 arcmin. A complex thermal-control system was implemented in order to remove at prime focus the majority of the 13 kW collected by the primary mirror and to keep optical surfaces and structures at ambient temperature, thus avoiding self-induced local seeing. A high-order adaptive-optics system with 1600 actuators corrects atmospheric seeing enabling diffraction limited imaging and spectroscopy. Five instruments, four of which are polarimeters, provide powerful diagnostic capability over a broad wavelength range covering the visible, near-infrared, and mid-infrared spectrum. New polarization-calibration strategies were developed to achieve the stringent polarization accuracy requirement of 5×10−4. Instruments can be combined and operated simultaneously in order to obtain a maximum of observational information. Observing time on DKIST is allocated through an open, merit-based proposal process. DKIST will be operated primarily in "service mode" and is expected to on average produce 3 PB of raw data per year. A newly developed data center located at the NSO Headquarters in Boulder will initially serve fully calibrated data to the international users community. Higher-level data products, such as physical parameters obtained from inversions of spectro-polarimetric data will be added as resources allow. Title: Spectropolarimetric Observations of an Arch Filament System with GREGOR Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.; Kuckein, C.; Kučera, A.; Schwartz, P.; Berkefeld, T.; Collados, M.; Denker, C.; Feller, A.; Hofmann, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; von der Lühe, O. Bibcode: 2019ASPC..526..217B Altcode: 2018arXiv180401789B We observed an arch filament system (AFS) in a sunspot group with the GREGOR Infrared Spectrograph attached to the GREGOR solar telescope. The AFS was located between the leading sunspot of negative polarity and several pores of positive polarity forming the following part of the sunspot group. We recorded five spectro-polarimetric scans of this region. The spectral range included the spectral lines Si I 1082.7 nm, He I 1083.0 nm, and Ca I 1083.9 nm. In this work we concentrate on the silicon line which is formed in the upper photosphere. The line profiles are inverted with the code 'Stokes Inversion based on Response functions' to obtain the magnetic field vector. The line-of-sight velocities are determined independently with a Fourier phase method. Maximum velocities are found close to the ends of AFS fibrils. These maximum values amount to 2.4 km s-1 next to the pores and to 4 km s-1 at the sunspot side. Between the following pores, we encounter an area of negative polarity that is decreasing during the five scans. We interpret this by new emerging positive flux in this area canceling out the negative flux. In summary, our findings confirm the scenario that rising magnetic flux tubes cause the AFS. Title: Photospheric Magnetic Fields of the Trailing Sunspots in Active Region NOAA 12396 Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer, C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2019ASPC..526..291V Altcode: 2018arXiv180507752V The solar magnetic field is responsible for all aspects of solar activity. Sunspots are the main manifestation of the ensuing solar activity. Combining high-resolution and synoptic observations has the ambition to provide a comprehensive description of the sunspot growth and decay processes. Active region NOAA 12396 emerged on 2015 August 3 and was observed three days later with the 1.5-meter GREGOR solar telescope on 2015 August 6. High-resolution spectropolarimetric data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the photospheric lines Si I λ1082.7 nm and Ca I λ1083.9 nm, together with the chromospheric He I λ1083.0 nm triplet. These near-infrared spectropolarimetric observations were complemented by synoptic line-of-sight magnetograms and continuum images of the Helioseismic and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Title: Status of the Daniel K. Inouye Solar Telescope: unraveling the mysteries the Sun. Authors: Rimmele, Thomas R.; Martinez Pillet, Valentin; Goode, Philip R.; Knoelker, Michael; Kuhn, Jeffrey Richard; Rosner, Robert; Casini, Roberto; Lin, Haosheng; von der Luehe, Oskar; Woeger, Friedrich; Tritschler, Alexandra; Fehlmann, Andre; Jaeggli, Sarah A.; Schmidt, Wolfgang; De Wijn, Alfred; Rast, Mark; Harrington, David M.; Sueoka, Stacey R.; Beck, Christian; Schad, Thomas A.; Warner, Mark; McMullin, Joseph P.; Berukoff, Steven J.; Mathioudakis, Mihalis; DKIST Team Bibcode: 2018AAS...23231601R Altcode: The 4m Daniel K. Inouye Solar Telescope (DKIST) currently under construction on Haleakala, Maui will be the world’s largest solar telescope. Designed to meet the needs of critical high resolution and high sensitivity spectral and polarimetric observations of the sun, this facility will perform key observations of our nearest star that matters most to humankind. DKIST’s superb resolution and sensitivity will enable astronomers to address many of the fundamental problems in solar and stellar astrophysics, including the origin of stellar magnetism, the mechanisms of coronal heating and drivers of the solar wind, flares, coronal mass ejections and variability in solar and stellar output. DKIST will also address basic research aspects of Space Weather and help improve predictive capabilities. In combination with synoptic observations and theoretical modeling DKIST will unravel the many remaining mysteries of the Sun.The construction of DKIST is progressing on schedule with 80% of the facility complete. Operations are scheduled to begin early 2020. DKIST will replace the NSO facilities on Kitt Peak and Sac Peak with a national facility with worldwide unique capabilities. The design allows DKIST to operate as a coronagraph. Taking advantage of its large aperture and infrared polarimeters DKIST will be capable to routinely measure the currently illusive coronal magnetic fields. The state-of-the-art adaptive optics system provides diffraction limited imaging and the ability to resolve features approximately 20 km on the Sun. Achieving this resolution is critical for the ability to observe magnetic structures at their intrinsic, fundamental scales. Five instruments will be available at the start of operations, four of which will provide highly sensitive measurements of solar magnetic fields throughout the solar atmosphere - from the photosphere to the corona. The data from these instruments will be distributed to the world wide community via the NSO/DKIST data center located in Boulder. We present examples of science objectives and provide an overview of the facility and project status, including the ongoing efforts of the community to develop the critical science plan for the first 2-3 years of operations. Title: Facilities for High Resolution Imaging of the Sun Authors: von der Lühe, Oskar Bibcode: 2018iss..confE..43V Altcode: The Sun is the only star where physical processes can be observed at their intrinsic spatial scales. Even though the Sun in a mere 150 million km from Earth, it is difficult to resolve fundamental processes in the solar atmosphere, because they occur at scales of the order of the kilometer. They can be observed only with telescopes which have apertures of several meters. The current state-of-the-art are solar telescopes with apertures of 1.5 m which resolve 50 km on the solar surface, soon to be superseded by telescopes with 4 m apertures with 20 km resolution. The US American 4 m DSI Solar Telescope is currently constructed on Maui, Hawaii, and is expected to have first light in 2020. The European solar community collaborates intensively to pursue the 4 m European Solar Telescope with a construction start in the Canaries early in the next decade. Solar telescopes with slightly smaller are also in the planning by the Russian, Indian and Chinese communities. In order to achieve a resolution which approaches the diffraction limit, all modern solar telescopes use adaptive optics which compensates virtually any scene on the solar disk. Multi-conjugate adaptive optics designed to compensate fields of the order on one minute of arc have been demonstrated and will become a facility feature of the new telescopes. The requirements for high precision spectro-polarimetry - about one part in 104 - makes continuous monitoring of (MC)AO performance and post-processing image reconstruction methods a necessity. Title: Franz-Ludwig Deubner (1934 - 2017) Authors: Fleck, Bernhard; von der Lühe, Oskar Bibcode: 2017SoPh..292..178F Altcode: No abstract at ADS Title: Simulation algorithm to model the visible tunable filter for the Daniel K. Inouye Solar Telescope Authors: Schubert, Matthias; Kentischer, Thomas; von der Lühe, Oskar Bibcode: 2017JATIS...3d5002S Altcode: The visible tunable filter is an imaging spectropolarimeter for solar observations in visible light. The instrument consists of several Fabry-Pérot interferometers (FPIs), a polarization modulator, and a prefilter. It will be one of the first light instruments for the Daniel K. Inouye Solar Telescope (DKIST) on Haleakaláa, Maui, Hawaii. We have developed simulation algorithms to describe the instrument and its impact on scientific observations. Our aim is to study the expected measurement accuracy and to test calibration algorithms. A well-known problem is the surface quality of the glass plates for each FPI. We developed algorithms to describe the influence of a surface microroughness, reflectivity, and figure errors of the individual FPI plates, and the expected total photon flux for scientific data acquisition. This tool is used to derive the limits for manufacturing processes to achieve the measurement accuracy required for science observations with DKIST. Title: Flows along arch filaments observed in the GRIS `very fast spectroscopic mode' Authors: González Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor Yabar, A.; Collados, M.; Verma, M.; Balthasar, H.; Diercke, A.; Fischer, C. E.; Gömöry, P.; Bello González, N.; Schlichenmaier, R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2017IAUS..327...28G Altcode: 2017arXiv170102206G A new generation of solar instruments provides improved spectral, spatial, and temporal resolution, thus facilitating a better understanding of dynamic processes on the Sun. High-resolution observations often reveal multiple-component spectral line profiles, e.g., in the near-infrared He i 10830 Å triplet, which provides information about the chromospheric velocity and magnetic fine structure. We observed an emerging flux region, including two small pores and an arch filament system, on 2015 April 17 with the `very fast spectroscopic mode' of the GREGOR Infrared Spectrograph (GRIS) situated at the 1.5-meter GREGOR solar telescope at Observatorio del Teide, Tenerife, Spain. We discuss this method of obtaining fast (one per minute) spectral scans of the solar surface and its potential to follow dynamic processes on the Sun. We demonstrate the performance of the `very fast spectroscopic mode' by tracking chromospheric high-velocity features in the arch filament system. Title: Clear widens the field for observations of the Sun with multi-conjugate adaptive optics Authors: Schmidt, Dirk; Gorceix, Nicolas; Goode, Philip R.; Marino, Jose; Rimmele, Thomas; Berkefeld, Thomas; Wöger, Friedrich; Zhang, Xianyu; Rigaut, François; von der Lühe, Oskar Bibcode: 2017A&A...597L...8S Altcode: The multi-conjugate adaptive optics (MCAO) pathfinder Clear on the New Solar Telescope in Big Bear Lake has provided the first-ever MCAO-corrected observations of the Sun that show a clearly and visibly widened corrected field of view compared to quasi-simultaneous observations with classical adaptive optics (CAO) correction. Clear simultaneously uses three deformable mirrors, each conjugated to a different altitude, to compensate for atmospheric turbulence. While the MCAO correction was most effective over an angle that is approximately three times wider than the angle that was corrected by CAO, the full 53'' field of view did benefit from MCAO correction. We further demonstrate that ground-layer-only correction is attractive for solar observations as a complementary flavor of adaptive optics for observational programs that require homogenous seeing improvement over a wide field rather than diffraction-limited resolution. We show illustrative images of solar granulation and of a sunspot obtained on different days in July 2016, and present a brief quantitative analysis of the generalized Fried parameters of the images.

The movies associated to Fig. 1 are available at http://www.aanda.org Title: Micro-meteorological contribution to the SHABAR seeing retrieval Authors: Hartogensis, Oscar; Hammerschlag, Robert; Sliepen, Guus; Sprung, Detlev; von der Lühe, Oskar; Collados, Manuel Bibcode: 2017psio.confE.102H Altcode: No abstract at ADS Title: Slipping reconnection in a solar flare observed in high resolution with the GREGOR solar telescope Authors: Sobotka, M.; Dudík, J.; Denker, C.; Balthasar, H.; Jurčák, J.; Liu, W.; Berkefeld, T.; Collados Vera, M.; Feller, A.; Hofmann, A.; Kneer, F.; Kuckein, C.; Lagg, A.; Louis, R. E.; von der Lühe, O.; Nicklas, H.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; Waldmann, T. Bibcode: 2016A&A...596A...1S Altcode: 2016arXiv160500464S A small flare ribbon above a sunspot umbra in active region 12205 was observed on November 7, 2014, at 12:00 UT in the blue imaging channel of the 1.5 m GREGOR telescope, using a 1 Å Ca II H interference filter. Context observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), the Solar Optical Telescope (SOT) onboard Hinode, and the Interface Region Imaging Spectrograph (IRIS) show that this ribbon is part of a larger one that extends through the neighboring positive polarities and also participates in several other flares within the active region. We reconstructed a time series of 140 s of Ca II H images by means of the multiframe blind deconvolution method, which resulted in spatial and temporal resolutions of 0.1″ and 1 s. Light curves and horizontal velocities of small-scale bright knots in the observed flare ribbon were measured. Some knots are stationary, but three move along the ribbon with speeds of 7-11 km s-1. Two of them move in the opposite direction and exhibit highly correlated intensity changes, which provides evidence of a slipping reconnection at small spatial scales.

Movies associated to Figs. 1 and 2 are available at http://www.aanda.org Title: Deep probing of the photospheric sunspot penumbra: no evidence of field-free gaps Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.; Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.; Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe, O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.; Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H. Bibcode: 2016A&A...596A...2B Altcode: 2016arXiv160708165B Context. Some models for the topology of the magnetic field in sunspot penumbrae predict regions free of magnetic fields or with only dynamically weak fields in the deep photosphere.
Aims: We aim to confirm or refute the existence of weak-field regions in the deepest photospheric layers of the penumbra.
Methods: We investigated the magnetic field at log τ5 = 0 is by inverting spectropolarimetric data of two different sunspots located very close to disk center with a spatial resolution of approximately 0.4-0.45''. The data have been recorded using the GRIS instrument attached to the 1.5-m solar telescope GREGOR at the El Teide observatory. The data include three Fe I lines around 1565 nm, whose sensitivity to the magnetic field peaks half a pressure scale height deeper than the sensitivity of the widely used Fe I spectral line pair at 630 nm. Before the inversion, the data were corrected for the effects of scattered light using a deconvolution method with several point spread functions.
Results: At log τ5 = 0 we find no evidence of regions with dynamically weak (B< 500 Gauss) magnetic fields in sunspot penumbrae. This result is much more reliable than previous investigations made on Fe I lines at 630 nm. Moreover, the result is independent of the number of nodes employed in the inversion, is independent of the point spread function used to deconvolve the data, and does not depend on the amount of stray light (I.e., wide-angle scattered light) considered. Title: Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.; Kuckein, C.; Kavka, J.; Kučera, A.; Schwartz, P.; Vašková, R.; Berkefeld, T.; Collados Vera, M.; Denker, C.; Feller, A.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016AN....337.1050B Altcode: 2016arXiv160901514B Arch filament systems occur in active sunspot groups, where a fibril structure connects areas of opposite magnetic polarity, in contrast to active region filaments that follow the polarity inversion line. We used the GREGOR Infrared Spectrograph (GRIS) to obtain the full Stokes vector in the spectral lines Si I λ1082.7 nm, He I λ1083.0 nm, and Ca I λ1083.9 nm. We focus on the near-infrared calcium line to investigate the photospheric magnetic field and velocities, and use the line core intensities and velocities of the helium line to study the chromospheric plasma. The individual fibrils of the arch filament system connect the sunspot with patches of magnetic polarity opposite to that of the spot. These patches do not necessarily coincide with pores, where the magnetic field is strongest. Instead, areas are preferred not far from the polarity inversion line. These areas exhibit photospheric downflows of moderate velocity, but significantly higher downflows of up to 30 km s-1 in the chromospheric helium line. Our findings can be explained with new emerging flux where the matter flows downward along the field lines of rising flux tubes, in agreement with earlier results. Title: Magnetic fields of opposite polarity in sunspot penumbrae Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.; Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld, T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.; Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar, A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier, K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M. Bibcode: 2016A&A...596A...4F Altcode: 2016arXiv160800513F Context. A significant part of the penumbral magnetic field returns below the surface in the very deep photosphere. For lines in the visible, a large portion of this return field can only be detected indirectly by studying its imprints on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe a narrow layer in the very deep photosphere, providing the possibility of directly measuring the orientation of magnetic fields close to the solar surface.
Aims: We study the topology of the penumbral magnetic field in the lower photosphere, focusing on regions where it returns below the surface.
Methods: We analyzed 71 spectropolarimetric datasets from Hinode and from the GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy of the infrared data after applying several reduction steps. Techniques of spectral inversion and forward synthesis were used to test the detection algorithm. We compared the morphology and the fractional penumbral area covered by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk center. We determined the amount of reversed-polarity and three-lobed Stokes V profiles in visible and infrared data of sunspots at various heliocentric angles. From the results, we computed center-to-limb variation curves, which were interpreted in the context of existing penumbral models.
Results: Observations in visible and near-infrared spectral lines yield a significant difference in the penumbral area covered by magnetic fields of opposite polarity. In the infrared, the number of reversed-polarity Stokes V profiles is smaller by a factor of two than in the visible. For three-lobed Stokes V profiles the numbers differ by up to an order of magnitude. Title: Horizontal flow fields in and around a small active region. The transition period between flux emergence and decay Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke, A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann, A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert, M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A...3V Altcode: 2016arXiv160507462V Context. The solar magnetic field is responsible for all aspects of solar activity. Thus, emergence of magnetic flux at the surface is the first manifestation of the ensuing solar activity.
Aims: Combining high-resolution and synoptic observations aims to provide a comprehensive description of flux emergence at photospheric level and of the growth process that eventually leads to a mature active region.
Methods: The small active region NOAA 12118 emerged on 2014 July 17 and was observed one day later with the 1.5-m GREGOR solar telescope on 2014 July 18. High-resolution time-series of blue continuum and G-band images acquired in the blue imaging channel (BIC) of the GREGOR Fabry-Pérot Interferometer (GFPI) were complemented by synoptic line-of-sight magnetograms and continuum images obtained with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Horizontal proper motions and horizontal plasma velocities were computed with local correlation tracking (LCT) and the differential affine velocity estimator (DAVE), respectively. Morphological image processing was employed to measure the photometric and magnetic area, magnetic flux, and the separation profile of the emerging flux region during its evolution.
Results: The computed growth rates for photometric area, magnetic area, and magnetic flux are about twice as high as the respective decay rates. The space-time diagram using HMI magnetograms of five days provides a comprehensive view of growth and decay. It traces a leaf-like structure, which is determined by the initial separation of the two polarities, a rapid expansion phase, a time when the spread stalls, and a period when the region slowly shrinks again. The separation rate of 0.26 km s-1 is highest in the initial stage, and it decreases when the separation comes to a halt. Horizontal plasma velocities computed at four evolutionary stages indicate a changing pattern of inflows. In LCT maps we find persistent flow patterns such as outward motions in the outer part of the two major pores, a diverging feature near the trailing pore marking the site of upwelling plasma and flux emergence, and low velocities in the interior of dark pores. We detected many elongated rapidly expanding granules between the two major polarities, with dimensions twice as large as the normal granules. Title: Upper chromospheric magnetic field of a sunspot penumbra: observations of fine structure Authors: Joshi, J.; Lagg, A.; Solanki, S. K.; Feller, A.; Collados, M.; Orozco Suárez, D.; Schlichenmaier, R.; Franz, M.; Balthasar, H.; Denker, C.; Berkefeld, T.; Hofmann, A.; Kiess, C.; Nicklas, H.; Pastor Yabar, A.; Rezaei, R.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A...8J Altcode: 2016arXiv160801988J
Aims: The fine-structure of the magnetic field in a sunspot penumbra in the upper chromosphere is to be explored and compared to that in the photosphere.
Methods: Spectropolarimetric observations with high spatial resolution were recorded with the 1.5-m GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric Hei triplet at 10 830 Å and the photospheric Sii 10 827.1 Å and Cai 10 833.4 Å spectral lines. The upper chromospheric magnetic field is obtained by inverting the Hei triplet assuming a Milne-Eddington-type model atmosphere. A height-dependent inversion was applied to the Sii 10 827.1 Å and Cai 10 833.4 Å lines to obtain the photospheric magnetic field.
Results: We find that the inclination of the magnetic field varies in the azimuthal direction in the photosphere and in the upper chromosphere. The chromospheric variations coincide remarkably well with the variations in the inclination of the photospheric field and resemble the well-known spine and interspine structure in the photospheric layers of penumbrae. The typical peak-to-peak variations in the inclination of the magnetic field in the upper chromosphere are found to be 10°-15°, which is roughly half the variation in the photosphere. In contrast, the magnetic field strength of the observed penumbra does not vary on small spatial scales in the upper chromosphere.
Conclusions: Thanks to the high spatial resolution of the observations that is possible with the GREGOR telescope at 1.08 microns, we find that the prominent small-scale fluctuations in the magnetic field inclination, which are a salient part of the property of sunspot penumbral photospheres, also persist in the chromosphere, although at somewhat reduced amplitudes. Such a complex magnetic configuration may facilitate penumbral chromospheric dynamic phenomena, such as penumbral micro-jets or transient bright dots. Title: Active region fine structure observed at 0.08 arcsec resolution Authors: Schlichenmaier, R.; von der Lühe, O.; Hoch, S.; Soltau, D.; Berkefeld, T.; Schmidt, D.; Schmidt, W.; Denker, C.; Balthasar, H.; Hofmann, A.; Strassmeier, K. G.; Staude, J.; Feller, A.; Lagg, A.; Solanki, S. K.; Collados, M.; Sigwarth, M.; Volkmer, R.; Waldmann, T.; Kneer, F.; Nicklas, H.; Sobotka, M. Bibcode: 2016A&A...596A...7S Altcode: 2016arXiv160707094S Context. The various mechanisms of magneto-convective energy transport determine the structure of sunspots and active regions.
Aims: We characterise the appearance of light bridges and other fine-structure details and elaborate on their magneto-convective nature.
Methods: We present speckle-reconstructed images taken with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the 486 nm and 589 nm bands. We estimate the spatial resolution from the noise characteristics of the image bursts and obtain 0.08″ at 589 nm. We describe structure details in individual best images as well as the temporal evolution of selected features.
Results: We find branched dark lanes extending along thin (≈1″) light bridges in sunspots at various heliocentric angles. In thick (≳ 2″) light bridges the branches are disconnected from the central lane and have a Y shape with a bright grain toward the umbra. The images reveal that light bridges exist on varying intensity levels and that their small-scale features evolve on timescales of minutes. Faint light bridges show dark lanes outlined by the surrounding bright features. Dark lanes are very common and are also found in the boundary of pores. They have a characteristic width of 0.1″ or smaller. Intergranular dark lanes of that width are seen in active region granulation.
Conclusions: We interpret our images in the context of magneto-convective simulations and findings: while central dark lanes in thin light bridges are elevated and associated with a density increase above upflows, the dark lane branches correspond to locations of downflows and are depressed relative to the adjacent bright plasma. Thick light bridges with central dark lanes show no projection effect. They have a flat elevated plateau that falls off steeply at the umbral boundary. There, Y-shaped filaments form as they do in the inner penumbra. This indicates the presence of inclined magnetic fields, meaning that the umbral magnetic field is wrapped around the convective light bridge. Title: Probing deep photospheric layers of the quiet Sun with high magnetic sensitivity Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González, M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.; Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.; Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.; Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier, K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.; Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.; Schmidt, D.; Kiess, C.; Nicklas, H. Bibcode: 2016A&A...596A...6L Altcode: 2016arXiv160506324L Context. Investigations of the magnetism of the quiet Sun are hindered by extremely weak polarization signals in Fraunhofer spectral lines. Photon noise, straylight, and the systematically different sensitivity of the Zeeman effect to longitudinal and transversal magnetic fields result in controversial results in terms of the strength and angular distribution of the magnetic field vector.
Aims: The information content of Stokes measurements close to the diffraction limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of spatial straylight and photon noise into account.
Methods: Highly sensitive full Stokes measurements of a quiet-Sun region at disk center in the deep photospheric Fe I lines in the 1.56 μm region were obtained with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise statistics and Stokes V asymmetries were analyzed and compared to a similar data set of the Hinode spectropolarimeter (SOT/SP). Simple diagnostics based directly on the shape and strength of the profiles were applied to the GRIS data. We made use of the magnetic line ratio technique, which was tested against realistic magneto-hydrodynamic simulations (MURaM).
Results: About 80% of the GRIS spectra of a very quiet solar region show polarimetric signals above a 3σ level. Area and amplitude asymmetries agree well with small-scale surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss range with some concentrations of kilo-Gauss fields.
Conclusions: The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4 are so far unique in the combination of high spatial resolution scans and high magnetic field sensitivity. Nevertheless, the unavoidable effect of spatial straylight and the resulting dilution of the weak Stokes profiles means that inversion techniques still bear a high risk of misinterpretating the data. Title: Flow and magnetic field properties in the trailing sunspots of active region NOAA 12396 Authors: Verma, M.; Denker, C.; Böhm, F.; Balthasar, H.; Fischer, C. E.; Kuckein, C.; Bello González, N.; Berkefeld, T.; Collados, M.; Diercke, A.; Feller, A.; González Manrique, S. J.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pator Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016AN....337.1090V Altcode: Improved measurements of the photospheric and chromospheric three-dimensional magnetic and flow fields are crucial for a precise determination of the origin and evolution of active regions. We present an illustrative sample of multi-instrument data acquired during a two-week coordinated observing campaign in August 2015 involving, among others, the GREGOR solar telescope (imaging and near-infrared spectroscopy) and the space missions Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS). The observations focused on the trailing part of active region NOAA 12396 with complex polarity inversion lines and strong intrusions of opposite polarity flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV spectral profiles in the photospheric Si I λ1082.7 nm line, the chromospheric He I λ1083.0 nm triplet, and the photospheric Ca I λ1083.9 nm line. Carefully calibrated GRIS scans of the active region provided maps of Doppler velocity and magnetic field at different atmospheric heights. We compare quick-look maps with those obtained with the ``Stokes Inversions based on Response functions'' (SIR) code, which furnishes deeper insight into the magnetic properties of the region. We find supporting evidence that newly emerging flux and intruding opposite polarity flux are hampering the formation of penumbrae, i.e., a penumbra fully surrounding a sunspot is only expected after cessation of flux emergence in proximity to the sunspots. Title: Three-dimensional structure of a sunspot light bridge Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.; Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A..59F Altcode: 2016arXiv161104803F Context. Active regions are the most prominent manifestations of solar magnetic fields; their generation and dissipation are fundamental problems in solar physics. Light bridges are commonly present during sunspot decay, but a comprehensive picture of their role in the removal of the photospheric magnetic field is still lacking.
Aims: We study the three-dimensional configuration of a sunspot, and in particular, its light bridge, during one of the last stages of its decay.
Methods: We present the magnetic and thermodynamical stratification inferred from full Stokes inversions of the photospheric Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope at the Observatorio del Teide, Tenerife, Spain. The analysis is complemented by a study of continuum images covering the disk passage of the active region, which are provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory.
Results: The sunspot shows a light bridge with penumbral continuum intensity that separates the central umbra from a smaller umbra. We find that in this region the magnetic field lines form a canopy with lower magnetic field strength in the inner part. The photospheric light bridge is dominated by gas pressure (high-β), as opposed to the surrounding umbra, where the magnetic pressure is higher. A convective flow is observed in the light bridge. This flow is able to bend the magnetic field lines and to produce field reversals. The field lines merge above the light bridge and become as vertical and strong as in the surrounding umbra. We conclude that this occurs because two highly magnetized regions approach each other during the sunspot evolution.

Movies associated to Figs. 2 and 13 are available at http://www.aanda.org Title: Inference of magnetic fields in the very quiet Sun Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.; Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.; Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.; González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.; Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman, T.; Volkmer, R. Bibcode: 2016A&A...596A...5M Altcode: 2018arXiv180410089M Context. Over the past 20 yr, the quietest areas of the solar surface have revealed a weak but extremely dynamic magnetism occurring at small scales (<500 km), which may provide an important contribution to the dynamics and energetics of the outer layers of the atmosphere. Understanding this magnetism requires the inference of physical quantities from high-sensitivity spectro-polarimetric data with high spatio-temporal resolution.
Aims: We present high-precision spectro-polarimetric data with high spatial resolution (0.4'') of the very quiet Sun at 1.56 μm obtained with the GREGOR telescope to shed some light on this complex magnetism.
Methods: We used inversion techniques in two main approaches. First, we assumed that the observed profiles can be reproduced with a constant magnetic field atmosphere embedded in a field-free medium. Second, we assumed that the resolution element has a substructure with either two constant magnetic atmospheres or a single magnetic atmosphere with gradients of the physical quantities along the optical depth, both coexisting with a global stray-light component.
Results: Half of our observed quiet-Sun region is better explained by magnetic substructure within the resolution element. However, we cannot distinguish whether this substructure comes from gradients of the physical parameters along the line of sight or from horizontal gradients (across the surface). In these pixels, a model with two magnetic components is preferred, and we find two distinct magnetic field populations. The population with the larger filling factor has very weak ( 150 G) horizontal fields similar to those obtained in previous works. We demonstrate that the field vector of this population is not constrained by the observations, given the spatial resolution and polarimetric accuracy of our data. The topology of the other component with the smaller filling factor is constrained by the observations for field strengths above 250 G: we infer hG fields with inclinations and azimuth values compatible with an isotropic distribution. The filling factors are typically below 30%. We also find that the flux of the two polarities is not balanced. From the other half of the observed quiet-Sun area 50% are two-lobed Stokes V profiles, meaning that 23% of the field of view can be adequately explained with a single constant magnetic field embedded in a non-magnetic atmosphere. The magnetic field vector and filling factor are reliable inferred in only 50% based on the regular profiles. Therefore, 12% of the field of view harbour hG fields with filling factors typically below 30%. At our present spatial resolution, 70% of the pixels apparently are non-magnetised. Title: Fitting peculiar spectral profiles in He I 10830Å absorption features Authors: González Manrique, S. J.; Kuckein, C.; Pastor Yabar, A.; Collados, M.; Denker, C.; Fischer, C. E.; Gömöry, P.; Diercke, A.; Bello González, N.; Schlichenmaier, R.; Balthasar, H.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016AN....337.1057G Altcode: 2016arXiv160300679G The new generation of solar instruments provides better spectral, spatial, and temporal resolution for a better understanding of the physical processes that take place on the Sun. Multiple-component profiles are more commonly observed with these instruments. Particularly, the He I 10830 Å triplet presents such peculiar spectral profiles, which give information on the velocity and magnetic fine structure of the upper chromosphere. The purpose of this investigation is to describe a technique to efficiently fit the two blended components of the He I 10830 Å triplet, which are commonly observed when two atmospheric components are located within the same resolution element. The observations used in this study were taken on 2015 April 17 with the very fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-m GREGOR solar telescope, located at the Observatorio del Teide, Tenerife, Spain. We apply a double-Lorentzian fitting technique using Levenberg-Marquardt least-squares minimization. This technique is very simple and much faster than inversion codes. Line-of-sight Doppler velocities can be inferred for a whole map of pixels within just a few minutes. Our results show sub- and supersonic downflow velocities of up to 32 km s-1 for the fast component in the vicinity of footpoints of filamentary structures. The slow component presents velocities close to rest. Title: Development of high reflectivity coatings for large format Fabry-Perot etalons Authors: Sigwarth, M.; Baumgartner, J.; Bell, A.; Cagnoli, G.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Kentischer, Th. J.; Kestner, B.; Kuschnir, P.; von der Lühe, O.; Pinard, L.; Michel, Ch.; Reichman, W. J.; Sassolas, B.; Scheiffelen, Th.; Schmidt, W. Bibcode: 2016SPIE.9908E..4FS Altcode: The Visible Tunable Filter (VTF) is a diffraction-limited narrowband tunable instrument for imaging spectropolarimetry in the wavelength range between 520 and 860 nm. It is based on large-format Fabry Perot. The instrument will be one of the first-light instruments of the 4m aperture Daniel K. Inoue Solar Telescope (DKIST). To provide a field of view of 1 arcmin and a spectral resolution λ/Δλ of about 100.000, the required free aperture of the Fabry Perot is 250mm. The high reflectivity coatings for the Etalon plates need to meet the specifications for the reflectivity over the entire wavelength range and preserve the plate figure specifications of better λ/300, and a micro roughness of < 0.4 nm rms. Coated surfaces with similar specifications have successfully been made for reflecting mirrors on thick substrates but not for larger format Fabry-Perot systems. Ion Beam Sputtering (IBS) based coatings provide stable, homogeneous, and smooth coatings. But IBS coatings also introduce stresses to the substrate that influence the plate figure in our case at the nm level. In a joint effort with an industry partner and a French CNRS research laboratory, we developed and tested processes on small and full size substrates, to provide coated Etalon plates to the required specifications. Zygo Extreme Precision Optics, Richmond, CA, USA, is polishing and figuring the substrates, doing the metrology and FE analysis. LMA (Laboratoire Matériaux Avancés, Lyon, France) is designing and making the IBS coatings and investigating the detailed behavior of the coatings and related processes. Both partners provide experience from manufacturing coated plane optics for gravitational wave detection experiments and EUV optics. The Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany is designing and building the VTF instrument and is leading the coating development. We present the characteristics of the coatings and the substrate processing concept, as well as results from tests on sample size and from full size substrate processing. We demonstrate that the tight specifications for a single Etalon can be reached. Title: End-to-end simulations of the visible tunable filter for the Daniel K. Inouye Solar Telescope Authors: Schmidt, Wolfgang; Schubert, Matthias; Ellwarth, Monika; Baumgartner, Jörg; Bell, Alexander; Fischer, Andreas; Halbgewachs, Clemens; Heidecke, Frank; Kentischer, Thomas; von der Lühe, Oskar; Scheiffelen, Thomas; Sigwarth, Michael Bibcode: 2016SPIE.9908E..4NS Altcode: 2016arXiv160706767S The Visible Tunable Filter (VTF) is a narrowband tunable filter system for imaging spectroscopy and spectropolarimetry based. The instrument will be one of the first-light instruments of the Daniel K. Inouye Solar Telescope that is currently under construction on Maui (Hawaii). The VTF is being developed by the Kiepenheuer Institut fuer Sonnenphysik in Freiburg as a German contribution to the DKIST. We perform end-to-end simulations of spectropolarimetric observations with the VTF to verify the science requirements of the instrument. The instrument is simulated with two Etalons, and with a single Etalon. The clear aperture of the Etalons is 250 mm, corresponding to a field of view with a diameter of 60 arcsec in the sky (42,000 km on the Sun). To model the large-scale figure errors we employ low-order Zernike polynomials (power and spherical aberration) with amplitudes of 2.5 nm RMS. We use an ideal polarization modulator with equal modulation coefficients of 3-1/2 for the polarization modulation We synthesize Stokes profiles of two iron lines (630.15 nm and 630.25 nm) and for the 854.2 nm line of calcium, for a range of magnetic field values and for several inclination angles. We estimated the photon noise on the basis of the DKIST and VTF transmission values, the atmospheric transmission and the spectral flux from the Sun. For the Fe 630.25 nm line, we obtain a sensitivity of 20 G for the longitudinal component and for 150 G for the transverse component, in agreement with the science requirements for the VTF. Title: Qualification of HEIDENHAIN linear encoders for picometer resolution metrology in VTF Etalons Authors: Halbgewachs, Clemens; Kentischer, Thomas J.; Sändig, Karsten; Baumgartner, Joerg; Bell, Alexander; Fischer, Andreas; Funk, Stefan; Heidecke, Frank; Scheiffelen, Thomas; Schmidt, Wolfgang; Spanner, Erwin; Speckbacher, Peter; von der Lühe, Oskar F. Bibcode: 2016SPIE.9908E..4HH Altcode: The Visible Tunable Filter (VTF) is a narrowband tunable instrument for imaging spectropolarimetry in the wavelength range between 520 and 870 nm. It is based on large-format Fabry Perots with a free aperture of 250 mm. The instrument will be one of the first-light instruments of the 4 m aperture Daniel K. Inoue Solar Telescope (DKIST) that is currently under construction on Maui (Hawaii). To provide stable and repeatable spectral scanning by tuning the air gap distance of the Etalons, a metrology system with 20 pm resolution and drift stability of better 100 pm per hour is needed. The integration of the metrology system must preserve the tight optical specifications of the Etalon plates. The HEIDENHAIN LIP 382 linear encoder system has a selected linear scale for low noise high signal interpolation. The signal period is 128nm and the interpolated signal from the sensor can be read out at 128 nm/ 14 bit = 7.8125 pm. To qualify the LIP 382 system for the VTF, we investigated the resolution and stability under nominal VTF operation conditions and verified a mounting concept for the sensor heads. We present results that demonstrate that the LIP 382 system fulfills the requirements for the VTF Etalons. We also present a design for the sensor head mounts. Title: Flows in and around Active Region NOAA12118 Observed with the GREGOR Solar Telescope and SDO/HMI Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke, A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann, A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert, M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016ASPC..504...29V Altcode: 2016arXiv160301109V Accurate measurements of magnetic and velocity fields in and around solar active regions are key to unlocking the mysteries of the formation and the decay of sunspots. High spatial resolution images and spectral sequences with a high cadence obtained with the GREGOR solar telescope give us an opportunity to scrutinize 3-D flow fields with local correlation tracking and imaging spectroscopy. We present GREGOR early science data acquired in 2014 July - August with the GREGOR Fabry-Pérot Interferometer and the Blue Imaging Channel. Time-series of blue continuum (λ 450.6 nm) images of the small active region NOAA 12118 were restored with the speckle masking technique to derive horizontal proper motions and to track the evolution of morphological changes. In addition, high-resolution observations are discussed in the context of synoptic data from the Solar Dynamics Observatory. Title: DKIST visible tunable filter control software: connecting the DKIST framework to OPC UA Authors: Bell, Alexander; Halbgewachs, Clemens; Kentischer, Thomas J.; Schmidt, Wolfgang; von der Lühe, Oskar; Sigwarth, Michael; Fischer, Andreas Bibcode: 2014SPIE.9152E..1DB Altcode: The Visible Tunable Filter (VTF) is a narrowband tunable filter system for imaging spectroscopy and spectropolarimetry based on large-format Fabry Perot interferometers that is currently built by the Kiepenheuer Institut fuer Sonnenphysik for the Daniel K. Inouye Solar Telescope (DKIST). The control software must handle around 30 motorised drives, 3 etalons, a polarizing modulator, a helium neon laser for system calibration, temperature controllers and a multitude of sensors. The VTF is foreseen as one of the DKISTs first-light instruments and should become operational in 2019. In the design of the control software we strongly separate between the high-level part interfacing to the DKIST common services framework (CSF) and the low-level control system software which guarantees real-time performance and synchronization to precision time protocol (PTP) based observatory time. For the latter we chose a programmable logic controller (PLC) from Beckhoff Automation GmbH which supports a wide set of input and output devices as well as distributed clocks for synchronizing signals down to the sub-microsecond level. In this paper we present the design of the required control system software as well as our work on extending the DKIST CSF to use the OPC Unified Architecture (OPC UA) standard which provides a cross-platform communication standard for process control and automation as an interface between the high-level software and the real-time control system. Title: GREGOR MCAO looking at the Sun Authors: Schmidt, Dirk; Berkefeld, Thomas; Heidecke, Frank; Fischer, Andreas; von der Lühe, Oskar; Soltau, Dirk Bibcode: 2014SPIE.9148E..1TS Altcode: A multi-conjugate adaptive optics systems has been deployed at the 1.5-meter solar telescope GREGOR for on-sun experiments of MCAO in November 2013. GREGOR MCAO incorporates three deformable mirrors (DMs) conjugate to 0, 8, and 25 km line of sight distance. Two correlating Shack-Hartmann wavefront sensor units are deployed: a high-order on-axis wavefront sensor (OA-WFS) with 10-cm subapertures and 10 arcsec field of view, and a low-order multi-direction wavefront sensor (MD-WFS) with 50-cm subapertures that sample the wavefront in 19 guide regions distributed over one arcminute. The MCAO loop was closed repeatedly in November '13, as well as in January and May '14. However, in particular strong static aberrations that were not removed well by the system, derogated the image in the MCAO compensated focal plane. GREGOR MCAO is now permanently installed and available for experiments that shall advance the development of solar MCAO. Title: A two-dimensional spectropolarimeter as a first-light instrument for the Daniel K. Inouye Solar Telescope Authors: Schmidt, Wolfgang; Bell, Alexander; Halbgewachs, Clemens; Heidecke, Frank; Kentischer, Thomas J.; von der Lühe, Oskar; Scheiffelen, Thomas; Sigwarth, Michael Bibcode: 2014SPIE.9147E..0ES Altcode: The Visible Tunable Filter (VTF) is a narrowband tunable filter system for imaging spectropolarimetry. The instrument will be one of the first-light instruments of the Daniel K. Inouye Solar Telescope (DKIST) that is currently under construction on Maui (Hawaii). The DKIST has a clear aperture of 4 meters. The VTF is being developed by the Kiepenheuer Institut für Sonnenphysik in Freiburg, as a German contribution to the DKIST. The VTF is designed as a diffraction-limited narrowband tunable instrument for Stokes spectro-polarimetry in the wavelength range between 520 and 860 nm. The instrument uses large-format Fabry-Perot interferometers (Etalons) as tunable monochromators with clear apertures of about 240 mm. To minimize the influence of gravity on the interferometer plates, the Fabry-Perots are placed horizontally. This implies a complex optical design and a three-dimensional support structure instead of a horizontal optical bench. The VTF has a field of view of one arc minute squared. With 4096x4096 pixel detectors, one pixel corresponds to an angle of 0.014" on the sky (10 x 10 km on the Sun). The spectral resolution is 6 pm at a wavelength of 600 nm. One 2Dspectrum with a polarimetric sensitivity of 5E-3 will be recorded within 13 seconds. The wavelength range of the VTF includes a number of important spectral lines for the measurement flows and magnetic fields in the atmosphere of the Sun. The VTF uses three identical large-format detectors, two for the polarimetric measurements, and one for broadband filtergrams. The main scientific observables of the VTF are Stokes polarimetric images to retrieve the magnetic field configuration of the observed area, Doppler images to measure the line-of-sight flow in the solar photosphere, and monochromatic intensity filtergrams to study higher layers of the solar atmosphere. Title: The Advanced Technology Solar Telescope: Science Drivers and Construction Status Authors: Rimmele, Thomas; Berger, Thomas; McMullin, Joseph; Keil, Stephen; Goode, Phil; Knoelker, Michael; Kuhn, Jeff; Rosner, Robert; Casini, Roberto; Lin, Haosheng; Woeger, Friedrich; von der Luehe, Oskar; Tritschler, Alexandra; Atst Team Bibcode: 2013EGUGA..15.6305R Altcode: The 4-meter Advance Technology Solar Telescope (ATST) currently under construction on the 3000 meter peak of Haleakala on Maui, Hawaii will be the world's most powerful solar telescope and the leading ground-based resource for studying solar magnetism. The solar atmosphere is permeated by a 'magnetic carpet' that constantly reweaves itself to control solar irradiance and its effects on Earth's climate, the solar wind, and space weather phenomena such as flares and coronal mass ejections. Precise measurement of solar magnetic fields requires a large-aperture solar telescope capable of resolving a few tens of kilometers on the solar surface. With its 4 meter aperture, the ATST will for the first time resolve magnetic structure at the intrinsic scales of plasma convection and turbulence. The ATST's ability to perform accurate and precise spectroscopic and polarimetric measurements of magnetic fields in all layers of the solar atmosphere, including accurate mapping of the elusive coronal magnetic fields, will be transformative in advancing our understanding of the magnetic solar atmosphere. The ATST will utilize the Sun as an important astro- and plasma-physics "laboratory" demonstrating key aspects of omnipresent cosmic magnetic fields. The ATST construction effort is led by the US National Solar Observatory. State-of-the-art instrumentation will be constructed by US and international partner institutions. The technical challenges the ATST is facing are numerous and include the design of the off-axis main telescope, the development of a high order adaptive optics system that delivers a corrected beam to the instrument laboratory, effective handling of the solar heat load on optical and structural elements, and minimizing scattered light to enable observations of the faint corona. The ATST project has transitioned from design and development to its construction phase. The project has awarded design and fabrication contracts for major telescope subsystems. Site construction has commenced following the successful conclusion of the site permitting process. Science goals and construction status of telescope and instrument systems will be discussed. Title: The GREGOR Solar Telescope on Tenerife Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.; Solanki, S. K.; Balthasar, H.; Bello González, N.; Berkefeld, T.; Collados Vera, M.; Hofmann, A.; Kneer, F.; Lagg, A.; Puschmann, K. G.; Schmidt, D.; Sobotka, M.; Soltau, D.; Strassmeier, K. G. Bibcode: 2012ASPC..463..365S Altcode: 2012arXiv1202.4289S 2011 was a successful year for the GREGOR project. The telescope was finally completed in May with the installation of the 1.5-meter primary mirror. The installation of the first-light focal plane instruments was completed by the end of the year. At the same time, the preparations for the installation of the high-order adaptive optics were finished, its integration to the telescope is scheduled for early 2012. This paper describes the telescope and its instrumentation in their present first-light configuration, and provides a brief overview of the science goals of GREGOR. Title: The GREGOR Solar Telescope Authors: Denker, C.; Lagg, A.; Puschmann, K. G.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K. G.; Volkmer, R.; von der Luehe, O.; Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.; Berkefeld, T.; Collados Vera, M.; Hofmann, A.; Kneer, F. Bibcode: 2012IAUSS...6E.203D Altcode: The 1.5-meter GREGOR solar telescope is a new facility for high-resolution observations of the Sun. The telescope is located at the Spanish Observatorio del Teide on Tenerife. The telescope incorporates advanced designs for a foldable-tent dome, an open steel-truss telescope structure, and active and passive means to minimize telescope and mirror seeing. Solar fine structure can be observed with a dedicated suite of instruments: a broad-band imaging system, the "GREGOR Fabry-Perot Interferometer", and the "Grating Infrared Spectrograph". All post-focus instruments benefit from a high-order (multi-conjugate) adaptive optics system, which enables observations close to the diffraction limit of the telescope. The inclusion of a spectrograph for stellar activity studies and the search for solar twins expands the scientific usage of the GREGOR to the nighttime domain. We report on the successful commissioning of the telescope until the end of 2011 and the first steps towards science verification in 2012. Title: Gregor@night: The future high-resolution stellar spectrograph for the GREGOR solar telescope Authors: Strassmeier, K. G.; Ilyin, I. V.; Woche, M.; Granzer, T.; Weber, M.; Weingrill, J.; Bauer, S. -M.; Popow, E.; Denker, C.; Schmidt, W.; von der Lühe, O.; Berdyugina, S.; Collados, M.; Koubsky, P.; Hackman, T.; Mantere, M. J. Bibcode: 2012AN....333..901S Altcode: We describe the future night-time spectrograph for the GREGOR solar telescope and present its science core projects. The spectrograph provides a 3-pixel resolution of up to R=87 000 in 45 échelle orders covering the wavelength range 390-900 nm with three grating settings. An iodine cell can be used for high-precision radial velocity work in the 500-630 nm range. The operation of the spectrograph and the telescope will be fully automated without the presence of humans during night-time and will be based on the successful STELLA control system. Future upgrades include a second optical camera for even higher spectral resolution, a Stokes-V polarimeter and a link to the laser-frequency comb at the Vacuum Tower Telescope. The night-time core projects are a study of the angular-momentum evolution of ``The Sun in Time'' and a continuation of our long-term Doppler imaging of active stars. Title: A retrospective of the GREGOR solar telescope in scientific literature Authors: Denker, C.; von der Lühe, O.; Feller, A.; Arlt, K.; Balthasar, H.; Bauer, S. -M.; Bello González, N.; Berkefeld, Th.; Caligari, P.; Collados, M.; Fischer, A.; Granzer, T.; Hahn, T.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Kentischer, T.; Klva{ňa, M.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.; Rendtel, J.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; Waldmann, T.; Wiehr, E.; Wittmann, A. D.; Woche, M. Bibcode: 2012AN....333..810D Altcode: 2012arXiv1210.3167D In this review, we look back upon the literature, which had the GREGOR solar telescope project as its subject including science cases, telescope subsystems, and post-focus instruments. The articles date back to the year 2000, when the initial concepts for a new solar telescope on Tenerife were first presented at scientific meetings. This comprehensive bibliography contains literature until the year 2012, i.e., the final stages of commissioning and science verification. Taking stock of the various publications in peer-reviewed journals and conference proceedings also provides the ``historical'' context for the reference articles in this special issue of Astronomische Nachrichten/Astronomical Notes. Title: The 1.5 meter solar telescope GREGOR Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.; Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.; Berkefeld, Th.; Collados, M.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.; Schmidt, D.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Waldmann , T. A. Bibcode: 2012AN....333..796S Altcode: The 1.5 m telescope GREGOR opens a new window to the understanding of solar small-scale magnetism. The first light instrumentation includes the Gregor Fabry Pérot Interferometer (GFPI), a filter spectro-polarimeter for the visible wavelength range, the GRating Infrared Spectro-polarimeter (GRIS) and the Broad-Band Imager (BBI). The excellent performance of the first two instruments has already been demonstrated at the Vacuum Tower Telescope. GREGOR is Europe's largest solar telescope and number 3 in the world. Its all-reflective Gregory design provides a large wavelength coverage from the near UV up to at least 5 microns. The field of view has a diameter of 150 arcsec. GREGOR is equipped with a high-order adaptive optics system, with a subaperture size of 10 cm, and a deformable mirror with 256 actuators. The science goals are focused on, but not limited to, solar magnetism. GREGOR allows us to measure the emergence and disappearance of magnetic flux at the solar surface at spatial scales well below 100 km. Thanks to its spectro-polarimetric capabilities, GREGOR will measure the interaction between the plasma flows, different kinds of waves, and the magnetic field. This will foster our understanding of the processes that heat the chromosphere and the outer layers of the solar atmosphere. Observations of the surface magnetic field at very small spatial scales will shed light on the variability of the solar brightness. Title: The GREGOR adaptive optics system Authors: Berkefeld , Th.; Schmidt, D.; Soltau, D.; von der Lühe, O.; Heidecke, F. Bibcode: 2012AN....333..863B Altcode: The new 1.5-m German solar telescope GREGOR at the Observatorio del Teide, Tenerife, is equipped with an integrated adaptive optics system. Although partly still in the commissioning phase, the system is already being used used for most science observations. It is designed to provide diffraction-limited observations in the visible-light regime for seeing better than 1.2 arcsec. We describe the AO system including the optical design, software, wavefront reconstruction, and performance. Title: Optical design of the new solar telescope GREGOR Authors: Soltau, D.; Volkmer, R.; von der Lühe, O.; Berkefeld, Th. Bibcode: 2012AN....333..847S Altcode: This article describes the considerations which led to the current optical design of the new 1.5 m solar telescope GREGOR. The result is Gregorian design with two real foci in the optical train. The telescope includes a relay optic with a pupil image used by a high order adaptive optics system (AO). The optical design is described in detail and performance characteristics are given. Finally we show some verification results which prove that - without atmospheric effects - the completed telescope reaches a diffraction limited performance. Title: Editors' note Authors: Denker, Carsten; Feller, Alex; Schmidt, Wolfgang; von der Lühe, Oskar Bibcode: 2012AN....333....1D Altcode: This topical issue of Astronomische Nachrichten/Astronomical Notes is a collection of reference articles covering the GREGOR solar telescope, its science capabilities, its subsystems, and its dedicated suite of instruments for high-resolution observations of the Sun. Because ground-based telescopes have life spans of several decades, it is only natural that they continuously reinvent themselves. Literally, the GREGOR telescope builds on the foundations of the venerable Gregory-Coudé Telescope (GCT) at Observatorio del Teide, Tenerife, Spain. Acknowledging the fact that new discoveries in observational solar physics are driven by larger apertures to collect more photons and to scrutinize the Sun in finer detail, the GCT was decommissioned and the building was made available to the GREGOR project. Title: Mechanical design of the solar telescope GREGOR Authors: Volkmer, R.; Eisenträger, P.; Emde, P.; Fischer, A.; von der Lühe, O.; Nicklas, H.; Soltau, D.; Schmidt, W.; Weis, U. Bibcode: 2012AN....333..816V Altcode: The mechanical structure of the GREGOR telescope was installed at the Observatorio del Teide, Tenerife, in 2004. New concepts for mounting and cooling of the 1.5-meter primary mirror were introduced. GREGOR is an open telescope, therefore the dome is completely open during observations to allow for air flushing through the open, but stiff telescope structure. Backside cooling system of the primary mirror keeps the mirror surface close to ambient temperature to prevent mirror seeing. The large collecting area of the primary mirror results in high energy density at the field stop at the prime focus of the primary which needs to be removed. The optical elements are supported by precision alignment systems and should provide a stable solar image at the optical lab. The coudé train can be evacuated and serves as a natural barrier between the outer environmental conditions and the air-conditioned optical laboratory with its sensitive scientific instrumentation. The telescope was successfully commissioned and will start its nominal operation during 2013. Title: The GREGOR Broad-Band Imager Authors: von der Lühe, O.; Volkmer, R.; Kentischer, T. J.; Geißler, R. Bibcode: 2012AN....333..894V Altcode: The design and characteristics of the Broad-Band Imager (BBI) of GREGOR are described. BBI covers the visible spectral range with two cameras simultaneously for a large field and with critical sampling at 390 nm, and it includes a mode for observing the pupil in a Foucault configuration. Samples of first-light observations are shown. Title: GRIS: The GREGOR Infrared Spectrograph Authors: Collados, M.; López, R.; Páez, E.; Hernández, E.; Reyes, M.; Calcines, A.; Ballesteros, E.; Díaz, J. J.; Denker, C.; Lagg, A.; Schlichenmaier, R.; Schmidt, W.; Solanki, S. K.; Strassmeier, K. G.; von der Lühe, O.; Volkmer, R. Bibcode: 2012AN....333..872C Altcode: This paper describes the main characteristics of GRIS (GREGOR Infrared Spectrograph), the grating spectrograph installed in the recently inaugurated (May 2012) 1.5-meter GREGOR telescope located at the Observatorio del Teide in Tenerife. The spectrograph has a standard Czerny-Turner configuration with parabolic collimator and camera mirrors that belong to the same conic surface. Although nothing prevents its use at visible wavelengths, the spectrograph will be initially used in combination with the infrared detector of the Tenerife Infrared Polarimeter (TIP-II) in standard spectroscopic mode as well as for spectropolarimetric measurements. Title: The visible tunable filtergraph for the ATST Authors: Kentischer, T. J.; Schmidt, W.; von der Lühe, O.; Sigwarth, M.; Bell, A.; Halbgewachs, C.; Fischer, A. Bibcode: 2012SPIE.8446E..77K Altcode: The Kiepenheuer-Institut will develop for the Advanced Technology Solar Telescope (ATST) a narrowband tunable filter system (Visible Tunable Filter, VTF) for imaging spectroscopy and spectropolarimetry based on large-format Fabry Perot interferometers. A major challenge for the realization of this instrument is the development of large-format Fabry-Perots with a free aperture of about 250 mm. The instrument will operate in the spectral range between 500 and 900 nm with access to a host of magnetically sensitive lines. The instrument is designed to match the diffraction limit of the 4m-aperture ATST and will be able to observe processes on the sun at spatial scales of 35 km. Its multi-line capability, together with a field of view of one arc minute, and the ability to measure polarization states of the incoming light allow to probe different layers of the solar atmosphere within a couple of seconds. The instrument is capable to vary the spectral sampling, the integration time, and the temporal cadence over a wide range without changing or compromising the opto-mechanical setup. This versatility gives unique possibilities to apply different measurement schemes to a variety of science questions. The ATST is a fully funded US project, with the VTF as the only non-US contribution, and is ready to start construction at the Haleakala summit. The VTF is foreseen as one of the ATST's firstlight instruments and should become operational in 2018. Title: Point Spread Function Estimation Using Speckle Reconstructions of Solar Surface Images Authors: Waldmann, T. A.; von der Lühe, O. Bibcode: 2010SoPh..267..217W Altcode: 2010SoPh..tmp..169W We present a method for retrieving the wavefront phase and estimating the point spread function using data from a single imaging channel. The method involves a set of short exposures and its speckle reconstruction of the observed object, and a parameterized point spread function model which is used in order to estimate the wavefront phase and the point spread function from the data. Estimates of the wavefront phase and the point spread function are computed by minimizing an error function with the method of simulated annealing. Results of tests with simulated data and with real data are shown. We conclude that the proposed technique provides reliable estimates of the instantaneous point spread function and is robust against noise. Title: Adaptive optics development at the German solar telescopes Authors: Berkefeld, Thomas; Soltau, Dirk; Schmidt, Dirk; von der Lühe, Oskar Bibcode: 2010ApOpt..49G.155B Altcode: No abstract at ADS Title: GREGOR telescope: start of commissioning Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S.; Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvana, M.; Kneer, F.; Lagg, A.; Popow, E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K. Bibcode: 2010SPIE.7733E..0KV Altcode: 2010SPIE.7733E..18V With the integration of a 1-meter Cesic primary mirror the GREGOR telescope pre-commissioning started. This is the first time, that the entire light path has seen sunlight. The pre-commissioning period includes testing of the main optics, adaptive optics, cooling system, and pointing system. This time was also used to install a near-infrared grating spectro-polarimeter and a 2D-spectropolarimeter for the visible range as first-light science instruments. As soon as the final 1.5 meter primary mirror is installed, commissioning will be completed, and an extended phase of science verification will follow. In the near future, GREGOR will be equipped with a multi-conjugate adaptive optics system that is presently under development at KIS. Title: GREGOR solar telescope: Design and status Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S. K.; Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Kneer, F.; Lagg, A.; Popow, E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K. G. Bibcode: 2010AN....331..624V Altcode: The integration and verification phase of the GREGOR telescope reached an important milestone with the installation of the interim 1 m SolarLite primary mirror. This was the first time that the entire light path had seen sunlight. Since then extensive testing of the telescope and its subsystems has been carried out. The integration and verification phase will culminate with the delivery and installation of the final 1.5 m Zerodur primary mirror in the summer of 2010. Observatory level tests and science verification will commence in the second half of 2010 and in 2011. This phase includes testing of the main optics, adaptive optics, cooling and pointing systems. In addition, assuming the viewpoint of a typical user, various observational modes of the GREGOR Fabry-Pérot Interferometer (GFPI), the Grating Infrared Spectrograph (GRIS), and high-speed camera systems will be tested to evaluate if they match the expectations and science requirements. This ensures that GREGOR will provide high-quality observations with its combination of (multi-conjugate) adaptive optics and advanced post-focus instruments. Routine observations are expected for 2012. Title: 2.1 Solar telescopes Authors: von der Lühe, Oskar Bibcode: 2010LanB...4A..210V Altcode: This document is part of Subvolume A 'Instruments and Methods' of Volume 4 'Astronomy, Astrophysics, and Cosmology' of Landolt-Börnstein - Group VI 'Astronomy and Astrophysics'. Title: Testbed for the multi-conjugate adaptive optics system of the solar telescope GREGOR Authors: Schmidt, Dirk; Berkefeld, T.; Heidecke, F.; von der Lühe, O.; Soltau, D. Bibcode: 2009SPIE.7439E..0XS Altcode: 2009SPIE.7439E..22S We present the latest concept of the multi-conjugate adaptive optics system for the 1.5-meter solar telescope Gregor. This system will employ three deformable mirrors in order to compensate for seeing introduced by the ground layer, and by shear winds in 5 and 15 km above the telescope ground. Thus, the compensated field of view will grow compared to ground layer compensation only. We describe the design and the used components and present a testbed which is used to improve control algorithms and to test all the components before installing them at the Gregor telescope. Title: History of solar telescopes Authors: von der Lühe, Oskar Bibcode: 2009ExA....25..193V Altcode: 2009ExA...tmp...36V Solar observations have been done with telescopes since their invention—already Galileo looked at the Sun. Despite the Sun’s unusual brightness, telescopes which specialize in solar observations are fairly recent, dating from the late nineteenth century onwards. Today, many solar telescopes have rather little in common with nighttime telescopes. They are adapted to high light flux, a limited range of declination, and to the specifications of solar spectrographs and polarimeters. This paper presents the history of the modern optical solar telescope on the ground and in space, the accompanying evolution of scientific capabilities, and a brief outlook into the future. Title: Comparison of geometrical mapping for ring diagram analysis Authors: Zaatri, Amel; Corbard, Thierry; Roth, Markus; González Hernández, Irene; von der Lühe, Oskar Bibcode: 2008JPhCS.118a2090Z Altcode: Mapping the solar surface is a crucial step in any local helioseismology technique. Because the acoustic waves propagate along great circles at the solar surface, it has been shown that these circles need to be used in the geometrical construction of the plane grid. We study different types of projections based on great circles for the calculation of sub-surface flows from ring diagram analysis of GONG data. Azimuthal equidistant projection, transverse cylindrical projection, gnomonic projection and stereographic projection produce almost the same velocity fields with standard patch sizes (15°x 15°). The difference between the four projections is more noticeable when larger patches (30° x 30°) are used. Title: Cross-spectral analysis of solar oscillation time series Authors: Schad, Ariane; Roth, Markus; Schelter, Björn; von der Lühe, Oskar; Timmer, Jens Bibcode: 2008JPhCS.118a2091S Altcode: Spatial leakage is an unavoidable artifact in the extraction of solar oscillation modes by spheric harmonic decomposition from the wavefield observed on the solar surface. The determination of solar frequencies by spectral analysis is therefore greatly affected by spatial leakage. Here we show in which way spatial leakage also influences the cross-spectra between different solar oscillation modes. Simulations show that spatial leakage induces significant coherences between oscillations of degree I and I + 2 with low azimuthal order m. Title: Calculation of Spectral Darkening and Visibility Functions for Solar Oscillations Authors: Nutto, C.; Roth, M.; Zhugzhda, Y.; Bruls, J.; von der Lühe, O. Bibcode: 2008SoPh..251..179N Altcode: 2008SoPh..tmp...63N; 2008arXiv0803.1228N Calculations of spectral darkening and visibility functions for the brightness oscillations of the Sun resulting from global solar oscillations are presented. This has been done for a broad range of the visible and infrared continuum spectrum. The procedure for the calculations of these functions includes the numerical computation of depth-dependent derivatives of the opacity caused by p modes in the photosphere. A radiative-transport code was used for this purpose to get the disturbances of the opacities from temperature and density fluctuations. The visibility and darkening functions are obtained for adiabatic oscillations under the assumption that the temperature disturbances are proportional to the undisturbed temperature of the photosphere. The latter assumption is the only way to explore any opacity effects since the eigenfunctions of p-mode oscillations have not been obtained so far. This investigation reveals that opacity effects have to be taken into account because they dominate the violet and infrared part of the spectrum. Because of this dominance, the visibility functions are negative for those parts of the spectrum. Furthermore, the darkening functions show a wavelength-dependent change of sign for some wavelengths owing to these opacity effects. However, the visibility and darkening functions under the assumptions used contradict the observations of global p-mode oscillations, but it is beyond doubt that the opacity effects influence the brightness fluctuations of the Sun resulting from global oscillations. Title: Speckle interferometry with adaptive optics corrected solar data Authors: Wöger, F.; von der Lühe, O.; Reardon, K. Bibcode: 2008A&A...488..375W Altcode: Context: Adaptive optics systems are used on several advanced solar telescopes to enhance the spatial resolution of the recorded data. In all cases, the correction remains only partial, requiring post-facto image reconstruction techniques such as speckle interferometry to achieve consistent, near-diffraction limited resolution.
Aims: This study investigates the reconstruction properties of the Kiepenheuer-Institut Speckle Interferometry Package (KISIP) code, with focus on its phase reconstruction capabilities and photometric accuracy. In addition, we analyze its suitability for real-time reconstruction.
Methods: We evaluate the KISIP program with respect to its scalability and the convergence of the implemented algorithms with dependence on several parameters, such as atmospheric conditions. To test the photometric accuracy of the final reconstruction, comparisons are made between simultaneous observations of the Sun using the ground-based Dunn Solar Telescope and the space-based Hinode/SOT telescope.
Results: The analysis shows that near real-time image reconstruction with high photometric accuracy of ground-based solar observations is possible, even for observations in which an adaptive optics system was utilized to obtain the speckle data. Title: Metallicity and ages of selected G-K giants Authors: Pasquini, L.; Döllinger, M.; Setiawan, J.; Hatzes, A.; Girardi, L.; da Silva, L.; de Medeiros, J. R.; Weiss, A.; von der Lühe, O. Bibcode: 2008mru..conf..132P Altcode: No abstract at ADS Title: Planets Around Active Stars Authors: Setiawan, J.; Weise, P.; Henning, Th.; Hatzes, A. P.; Pasquini, L.; da Silva, L.; Girardi, L.; von der Lühe, O.; Döllinger, M. P.; Weiss, A.; Biazzo, K. Bibcode: 2008psa..conf..201S Altcode: 2007arXiv0704.2145S We present the results of radial velocity measurements of two samples of active stars. The first sample contains field G and K giants across the Red Giant Branch, whereas the second sample consists of nearby young stars (d < 150 pc) with ages between 10 and 300 Myrs. The radial velocity monitoring program has been carried out with FEROS at 1.52 m ESO telescope (1999 - 2002) and continued since 2003 at 2.2 m MPG/ESO telescope. We observed stellar radial velocity variations which originate either from the stellar activity or the presence of stellar/substellar companions. By means of a bisector technique we are able to distinguish the sources of the radial velocity variation. Among them we found few candidates of planetary companions, both of young stars and G-K giants sample. Title: Field dependent amplitude calibration of adaptive optics supported solar speckle imaging Authors: Wöger, Friedrich; von der Lühe, Oskar Bibcode: 2007ApOpt..46.8015W Altcode: Adaptive optics supported solar speckle imaging requires the calibration of the source's Fourier amplitudes with the transfer function of atmosphere and optics. We present analytical models for the relevant transfer functions of an adaptive optics systems. The models include the effect of an arbitrary correction as well as anisoplanatism. The proposed models have been compared with observational data using measurements of α-Orionis and of the solar surface delivering both a direct and indirect method (using the spectral ratio technique) for validation. We find that measurements and model agree to a satisfactory degree. Title: GREGOR: the New German Solar Telescope Authors: Balthasar, H.; von der Lühe, O.; Kneer, F.; Staude, J.; Volkmer, R.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Nicklas, H.; Popow, E.; Puschmann, K.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K.; Wittmann, A. Bibcode: 2007ASPC..368..605B Altcode: 2007arXiv0704.2693B GREGOR is a new open solar telescope with an aperture of 1.5 m. It replaces the former 45-cm Gregory Coudé telescope on the Canary island Tenerife. The optical concept is that of a double Gregory system. The main and the elliptical mirrors are made from a silicon-carbide material with high thermal conductivity. This is important to keep the mirrors on the ambient temperature avoiding local turbulence. GREGOR will be equipped with an adaptive optics system. The new telescope will be ready for operation in 2008. Post-focus instruments in the first stage will be a spectrograph for polarimetry in the near infrared and a 2-dimensional spectrometer based on Fabry-Pérot interferometers for the visible. Title: AMBER, the near-infrared spectro-interferometric three-telescope VLTI instrument Authors: Petrov, R. G.; Malbet, F.; Weigelt, G.; Antonelli, P.; Beckmann, U.; Bresson, Y.; Chelli, A.; Dugué, M.; Duvert, G.; Gennari, S.; Glück, L.; Kern, P.; Lagarde, S.; Le Coarer, E.; Lisi, F.; Millour, F.; Perraut, K.; Puget, P.; Rantakyrö, F.; Robbe-Dubois, S.; Roussel, A.; Salinari, P.; Tatulli, E.; Zins, G.; Accardo, M.; Acke, B.; Agabi, K.; Altariba, E.; Arezki, B.; Aristidi, E.; Baffa, C.; Behrend, J.; Blöcker, T.; Bonhomme, S.; Busoni, S.; Cassaing, F.; Clausse, J. -M.; Colin, J.; Connot, C.; Delboulbé, A.; Domiciano de Souza, A.; Driebe, T.; Feautrier, P.; Ferruzzi, D.; Forveille, T.; Fossat, E.; Foy, R.; Fraix-Burnet, D.; Gallardo, A.; Giani, E.; Gil, C.; Glentzlin, A.; Heiden, M.; Heininger, M.; Hernandez Utrera, O.; Hofmann, K. -H.; Kamm, D.; Kiekebusch, M.; Kraus, S.; Le Contel, D.; Le Contel, J. -M.; Lesourd, T.; Lopez, B.; Lopez, M.; Magnard, Y.; Marconi, A.; Mars, G.; Martinot-Lagarde, G.; Mathias, P.; Mège, P.; Monin, J. -L.; Mouillet, D.; Mourard, D.; Nussbaum, E.; Ohnaka, K.; Pacheco, J.; Perrier, C.; Rabbia, Y.; Rebattu, S.; Reynaud, F.; Richichi, A.; Robini, A.; Sacchettini, M.; Schertl, D.; Schöller, M.; Solscheid, W.; Spang, A.; Stee, P.; Stefanini, P.; Tallon, M.; Tallon-Bosc, I.; Tasso, D.; Testi, L.; Vakili, F.; von der Lühe, O.; Valtier, J. -C.; Vannier, M.; Ventura, N. Bibcode: 2007A&A...464....1P Altcode: Context: Optical long-baseline interferometry is moving a crucial step forward with the advent of general-user scientific instruments that equip large aperture and hectometric baseline facilities, such as the Very Large Telescope Interferometer (VLTI).
Aims: AMBER is one of the VLTI instruments that combines up to three beams with low, moderate and high spectral resolutions in order to provide milli-arcsecond spatial resolution for compact astrophysical sources in the near-infrared wavelength domain. Its main specifications are based on three key programs on young stellar objects, active galactic nuclei central regions, masses, and spectra of hot extra-solar planets.
Methods: These key science goals led to scientific specifications, which were used to propose and then validate the instrument concept. AMBER uses single-mode fibers to filter the entrance signal and to reach highly accurate, multiaxial three-beam combination, yielding three baselines and a closure phase, three spectral dispersive elements, and specific self-calibration procedures.
Results: The AMBER measurements yield spectrally dispersed calibrated visibilities, color-differential complex visibilities, and a closure phase allows astronomers to contemplate rudimentary imaging and highly accurate visibility and phase differential measurements. AMBER was installed in 2004 at the Paranal Observatory. We describe here the present implementation of the instrument in the configuration with which the astronomical community can access it.
Conclusions: .After two years of commissioning tests and preliminary observations, AMBER has produced its first refereed publications, allowing assessment of its scientific potential. Title: Estimating the solar meridional circulation by normal mode decomposition Authors: Krieger, L.; Roth, M.; von der Lühe, O. Bibcode: 2007AN....328..252K Altcode: 2008arXiv0811.0693K The objective of this article is to use Fourier-Hankel decomposition as suggested earlier by Braun & Fan (1998) to estimate the integrated horizontal meridional flow velocity as a function of mode penetration depth, and to find ways of potentially improve this technique. We use a time series of 43200 (30 days) consecutive full-disk Dopplergrams obtained by the MDI (Michelson Doppler Imager) instrument aboard the SOHO (Solar Heliospheric Observatory) spacecraft in April 1999. We find averaged meridional flow estimates of 15 m/s for modes with a penetration depth in the upper 20 Mm of the solar convection zone. This reproduces the results of the earlier investigations. Moreover we conclude that this method has the potential to become a new technique to measure the meridional circulation in the deep convection zone, if some improvements will be applied. Title: An asymmetry detected in the disk of κ Canis Majoris with AMBER/VLTI Authors: Meilland, A.; Millour, F.; Stee, P.; Domiciano de Souza, A.; Petrov, R. G.; Mourard, D.; Jankov, S.; Robbe-Dubois, S.; Spang, A.; Aristidi, E.; Antonelli, P.; Beckmann, U.; Bresson, Y.; Chelli, A.; Dugué, M.; Duvert, G.; Gennari, S.; Glück, L.; Kern, P.; Lagarde, S.; Le Coarer, E.; Lisi, F.; Malbet, F.; Perraut, K.; Puget, P.; Rantakyrö, F.; Roussel, A.; Tatulli, E.; Weigelt, G.; Zins, G.; Accardo, M.; Acke, B.; Agabi, K.; Altariba, E.; Arezki, B.; Baffa, C.; Behrend, J.; Blöcker, T.; Bonhomme, S.; Busoni, S.; Cassaing, F.; Clausse, J. -M.; Colin, J.; Connot, C.; Delboulbé, A.; Driebe, T.; Feautrier, P.; Ferruzzi, D.; Forveille, T.; Fossat, E.; Foy, R.; Fraix-Burnet, D.; Gallardo, A.; Giani, E.; Gil, C.; Glentzlin, A.; Heiden, M.; Heininger, M.; Hernandez Utrera, O.; Hofmann, K. -H.; Kamm, D.; Kiekebusch, M.; Kraus, S.; Le Contel, D.; Le Contel, J. -M.; Lesourd, T.; Lopez, B.; Lopez, M.; Magnard, Y.; Marconi, A.; Mars, G.; Martinot-Lagarde, G.; Mathias, P.; Mège, P.; Monin, J. -L.; Mouillet, D.; Nussbaum, E.; Ohnaka, K.; Pacheco, J.; Perrier, C.; Rabbia, Y.; Rebattu, S.; Reynaud, F.; Richichi, A.; Robini, A.; Sacchettini, M.; Schertl, D.; Schöller, M.; Solscheid, W.; Stefanini, P.; Tallon, M.; Tallon-Bosc, I.; Tasso, D.; Testi, L.; Vakili, F.; von der Lühe, O.; Valtier, J. -C.; Vannier, M.; Ventura, N. Bibcode: 2007A&A...464...73M Altcode: 2006astro.ph.11563M Aims:We study the geometry and kinematics of the circumstellar environment of the Be star κ CMa in the Brγ emission line and its nearby continuum.
Methods: We use the AMBER/VLTI instrument operating in the K band, which provides a spatial resolution of about 6 mas with a spectral resolution of 1500, to study the kinematics within the disk and to infer its rotation law. To obtain more kinematical constraints we also use a high spectral resolution Paβ line profile obtain in December 2005 at the Observatorio do Pico do Dios, Brazil and we compile V/R line profile variations and spectral energy distribution data points from the literature.
Results: Using differential visibilities and differential phases across the Brγ line we detect an asymmetry in the disk. Moreover, we found that κ CMa seems difficult to fit within the classical scenario for Be stars, illustrated recently by α Arae observations, i.e. a fast rotating B star close to its breakup velocity surrounded by a Keplerian circumstellar disk with an enhanced polar wind. We discuss the possibility that κ CMa is a critical rotator with a Keplerian rotating disk and examine whether if the detected asymmetry can be interpreted within the “one-armed” viscous disk framework.

Based on observations collected at the European Southern Observatory, Paranal, Chile, within the guaranteed time programme 074.A-9025(A). Title: Constraining the wind launching region in Herbig Ae stars: AMBER/VLTI spectroscopy of HD 104237 Authors: Tatulli, E.; Isella, A.; Natta, A.; Testi, L.; Marconi, A.; Malbet, F.; Stee, P.; Petrov, R. G.; Millour, F.; Chelli, A.; Duvert, G.; Antonelli, P.; Beckmann, U.; Bresson, Y.; Dugué, M.; Gennari, S.; Glück, L.; Kern, P.; Lagarde, S.; Le Coarer, E.; Lisi, F.; Perraut, K.; Puget, P.; Rantakyrö, F.; Robbe-Dubois, S.; Roussel, A.; Weigelt, G.; Zins, G.; Accardo, M.; Acke, B.; Agabi, K.; Altariba, E.; Arezki, B.; Aristidi, E.; Baffa, C.; Behrend, J.; Blöcker, T.; Bonhomme, S.; Busoni, S.; Cassaing, F.; Clausse, J. -M.; Colin, J.; Connot, C.; Delboulbé, A.; Domiciano de Souza, A.; Driebe, T.; Feautrier, P.; Ferruzzi, D.; Forveille, T.; Fossat, E.; Foy, R.; Fraix-Burnet, D.; Gallardo, A.; Giani, E.; Gil, C.; Glentzlin, A.; Heiden, M.; Heininger, M.; Hernandez Utrera, O.; Hofmann, K. -H.; Kamm, D.; Kiekebusch, M.; Kraus, S.; Le Contel, D.; Le Contel, J. -M.; Lesourd, T.; Lopez, B.; Lopez, M.; Magnard, Y.; Mars, G.; Martinot-Lagarde, G.; Mathias, P.; Mège, P.; Monin, J. -L.; Mouillet, D.; Mourard, D.; Nussbaum, E.; Ohnaka, K.; Pacheco, J.; Perrier, C.; Rabbia, Y.; Rebattu, S.; Reynaud, F.; Richichi, A.; Robini, A.; Sacchettini, M.; Schertl, D.; Schöller, M.; Solscheid, W.; Spang, A.; Stefanini, P.; Tallon, M.; Tallon-Bosc, I.; Tasso, D.; Vakili, F.; von der Lühe, O.; Valtier, J. -C.; Vannier, M.; Ventura, N. Bibcode: 2007A&A...464...55T Altcode: 2006astro.ph..6684T Aims:We investigate the origin of the Brγ emission of the Herbig Ae star HD 104237 on Astronomical Unit (AU) scales.
Methods: Using AMBER/VLTI at a spectral resolution R=1500 we spatially resolve the emission in both the Brγ line and the adjacent continuum.
Results: The visibility does not vary between the continuum and the Brγ line, even though the line is strongly detected in the spectrum, with a peak intensity 35% above the continuum. This demonstrates that the line and continuum emission have similar size scales. We assume that the K-band continuum excess originates in a “puffed-up” inner rim of the circumstellar disk, and discuss the likely origin of Brγ.
Conclusions: .We conclude that this emission most likely arises from a compact disk wind, launched from a region 0.2-0.5 AU from the star, with a spatial extent similar to that of the near infrared continuum emission region, i.e., very close to the inner rim location.

Based on observations collected at the European Southern Observatory, Paranal, Chile, within the guaranteed time programme 074.A-9025(A). Title: Disk and wind interaction in the young stellar object MWC 297 spatially resolved with AMBER/VLTI Authors: Malbet, F.; Benisty, M.; de Wit, W. -J.; Kraus, S.; Meilland, A.; Millour, F.; Tatulli, E.; Berger, J. -P.; Chesneau, O.; Hofmann, K. -H.; Isella, A.; Natta, A.; Petrov, R. G.; Preibisch, T.; Stee, P.; Testi, L.; Weigelt, G.; Antonelli, P.; Beckmann, U.; Bresson, Y.; Chelli, A.; Dugué, M.; Duvert, G.; Gennari, S.; Glück, L.; Kern, P.; Lagarde, S.; Le Coarer, E.; Lisi, F.; Perraut, K.; Puget, P.; Rantakyrö, F.; Robbe-Dubois, S.; Roussel, A.; Zins, G.; Accardo, M.; Acke, B.; Agabi, K.; Altariba, E.; Arezki, B.; Aristidi, E.; Baffa, C.; Behrend, J.; Blöcker, T.; Bonhomme, S.; Busoni, S.; Cassaing, F.; Clausse, J. -M.; Colin, J.; Connot, C.; Delboulbé, A.; Domiciano de Souza, A.; Driebe, T.; Feautrier, P.; Ferruzzi, D.; Forveille, T.; Fossat, E.; Foy, R.; Fraix-Burnet, D.; Gallardo, A.; Giani, E.; Gil, C.; Glentzlin, A.; Heiden, M.; Heininger, M.; Hernandez Utrera, O.; Kamm, D.; Kiekebusch, M.; Le Contel, D.; Le Contel, J. -M.; Lesourd, T.; Lopez, B.; Lopez, M.; Magnard, Y.; Marconi, A.; Mars, G.; Martinot-Lagarde, G.; Mathias, P.; Mège, P.; Monin, J. -L.; Mouillet, D.; Mourard, D.; Nussbaum, E.; Ohnaka, K.; Pacheco, J.; Perrier, C.; Rabbia, Y.; Rebattu, S.; Reynaud, F.; Richichi, A.; Robini, A.; Sacchettini, M.; Schertl, D.; Schöller, M.; Solscheid, W.; Spang, A.; Stefanini, P.; Tallon, M.; Tallon-Bosc, I.; Tasso, D.; Vakili, F.; von der Lühe, O.; Valtier, J. -C.; Vannier, M.; Ventura, N. Bibcode: 2007A&A...464...43M Altcode: 2005astro.ph.10350M The young stellar object MWC 297 is an embedded B1.5Ve star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. This object has been observed with the VLT interferometer equipped with the AMBER instrument during its first commissioning run. AMBER/VLTI is currently the only near infrared interferometer that can observe spectrally dispersed visibilities. MWC 297 has been spatially resolved in the continuum with a visibility of 0.50+0.08-0.10 as well as in the Brγ emission line where the visibility decreases to 0.33±0.06. This change in the visibility with wavelength can be interpreted by the presence of an optically thick disk responsible for the visibility in the continuum and of a stellar wind traced by the Brγ emission line and whose apparent size is 40% larger. We validate this interpretation by building a model of the stellar environment that combines a geometrically thin, optically thick accretion disk model consisting of gas and dust, and a latitude-dependent stellar wind outflowing above the disk surface. The continuum emission and visibilities obtained from this model are fully consistent with the interferometric AMBER data. They agree also with existing optical, near-infrared spectra and other broad-band near-infrared interferometric visibilities. We also reproduce the shape of the visibilities in the Brγ line as well as the profile of this line obtained at an higher spectral resolution with the VLT/ISAAC spectrograph, and those of the Hα and Hβ lines. The disk and wind models yield a consistent inclination of the system of approximately 20°. A picture emerges in which MWC 297 is surrounded by an equatorial flat disk that is possibly still accreting and an outflowing wind that has a much higher velocity in the polar region than at the equator. The AMBER/VLTI unique capability of measuring spectral visibilities therefore allows us for the first time to compare the apparent geometry of a wind with the disk structure in a young stellar system.

Based on observations collected at the European Southern Observatory, Paranal, Chile, within the commissioning programme 60.A-9054(A). Title: Near-infrared interferometry of η Carinae with spectral resolutions of 1 500 and 12 000 using AMBER/VLTI Authors: Weigelt, G.; Kraus, S.; Driebe, T.; Petrov, R. G.; Hofmann, K. -H.; Millour, F.; Chesneau, O.; Schertl, D.; Malbet, F.; Hillier, J. D.; Gull, T.; Davidson, K.; Domiciano de Souza, A.; Antonelli, P.; Beckmann, U.; Bresson, Y.; Chelli, A.; Dugué, M.; Duvert, G.; Gennari, S.; Glück, L.; Kern, P.; Lagarde, S.; Le Coarer, E.; Lisi, F.; Perraut, K.; Puget, P.; Rantakyrö, F.; Robbe-Dubois, S.; Roussel, A.; Tatulli, E.; Zins, G.; Accardo, M.; Acke, B.; Agabi, K.; Altariba, E.; Arezki, B.; Aristidi, E.; Baffa, C.; Behrend, J.; Blöcker, T.; Bonhomme, S.; Busoni, S.; Cassaing, F.; Clausse, J. -M.; Colin, J.; Connot, C.; Delboulbé, A.; Feautrier, P.; Ferruzzi, D.; Forveille, T.; Fossat, E.; Foy, R.; Fraix-Burnet, D.; Gallardo, A.; Giani, E.; Gil, C.; Glentzlin, A.; Heiden, M.; Heininger, M.; Hernandez Utrera, O.; Kamm, D.; Kiekebusch, M.; Le Contel, D.; Le Contel, J. -M.; Lesourd, T.; Lopez, B.; Lopez, M.; Magnard, Y.; Marconi, A.; Mars, G.; Martinot-Lagarde, G.; Mathias, P.; Mège, P.; Monin, J. -L.; Mouillet, D.; Mourard, D.; Nussbaum, E.; Ohnaka, K.; Pacheco, J.; Perrier, C.; Rabbia, Y.; Rebattu, S.; Reynaud, F.; Richichi, A.; Robini, A.; Sacchettini, M.; Schöller, M.; Solscheid, W.; Spang, A.; Stee, P.; Stefanini, P.; Tallon, M.; Tallon-Bosc, I.; Tasso, D.; Testi, L.; Vakili, F.; von der Lühe, O.; Valtier, J. -C.; Vannier, M.; Ventura, N.; Weis, K.; Wittkowski, M. Bibcode: 2007A&A...464...87W Altcode: 2006astro.ph..9715W Aims: We present the first NIR spectro-interferometry of the LBV η Carinae. The observations were performed with the AMBER instrument of the ESO Very Large Telescope Interferometer (VLTI) using baselines from 42 to 89 m. The aim of this work is to study the wavelength dependence of η Car's optically thick wind region with a high spatial resolution of 5 mas (11 AU) and high spectral resolution.
Methods: The observations were carried out with three 8.2 m Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1500 (MR-K mode) and 12 000 (HR-K mode). The MR-K observations were performed in the wavelength range around both the He I 2.059 μm and the Brγ 2.166 μm emission lines, the HR-K observations only in the Brγ line region.
Results: The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of η Car. In the K-band continuum, a diameter of 4.0±0.2 mas (Gaussian FWHM, fit range 28-89 m baseline length) was measured for η Car's optically thick wind region. If we fit Hillier et al. (2001, ApJ, 553, 837) model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.2, 6.5 and 9.6 mas in the 2.17 μm continuum, the He I, and the Brγ emission lines, respectively. In the continuum near the Brγ line, an elongation along a position angle of 120°±15° was found, consistent with previous VINCI/VLTI measurements by van Boekel et al. (2003, A&A, 410, L37). We compare the measured visibilities with predictions of the radiative transfer model of Hillier et al. (2001), finding good agreement. Furthermore, we discuss the detectability of the hypothetical hot binary companion. For the interpretation of the non-zero differential and closure phases measured within the Brγ line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.

Based on observations collected at the European Southern Observatory, Paranal, Chile, within the AMBER guaranteed time programme 074.A-9025 and the VLTI science demonstration programme 074.A-9024. Title: Interferometric data reduction with AMBER/VLTI. Principle, estimators, and illustration Authors: Tatulli, E.; Millour, F.; Chelli, A.; Duvert, G.; Acke, B.; Hernandez Utrera, O.; Hofmann, K. -H.; Kraus, S.; Malbet, F.; Mège, P.; Petrov, R. G.; Vannier, M.; Zins, G.; Antonelli, P.; Beckmann, U.; Bresson, Y.; Dugué, M.; Gennari, S.; Glück, L.; Kern, P.; Lagarde, S.; Le Coarer, E.; Lisi, F.; Perraut, K.; Puget, P.; Rantakyrö, F.; Robbe-Dubois, S.; Roussel, A.; Weigelt, G.; Accardo, M.; Agabi, K.; Altariba, E.; Arezki, B.; Aristidi, E.; Baffa, C.; Behrend, J.; Blöcker, T.; Bonhomme, S.; Busoni, S.; Cassaing, F.; Clausse, J. -M.; Colin, J.; Connot, C.; Delboulbé, A.; Domiciano de Souza, A.; Driebe, T.; Feautrier, P.; Ferruzzi, D.; Forveille, T.; Fossat, E.; Foy, R.; Fraix-Burnet, D.; Gallardo, A.; Giani, E.; Gil, C.; Glentzlin, A.; Heiden, M.; Heininger, M.; Kamm, D.; Kiekebusch, M.; Le Contel, D.; Le Contel, J. -M.; Lesourd, T.; Lopez, B.; Lopez, M.; Magnard, Y.; Marconi, A.; Mars, G.; Martinot-Lagarde, G.; Mathias, P.; Monin, J. -L.; Mouillet, D.; Mourard, D.; Nussbaum, E.; Ohnaka, K.; Pacheco, J.; Perrier, C.; Rabbia, Y.; Rebattu, S.; Reynaud, F.; Richichi, A.; Robini, A.; Sacchettini, M.; Schertl, D.; Schöller, M.; Solscheid, W.; Spang, A.; Stee, P.; Stefanini, P.; Tallon, M.; Tallon-Bosc, I.; Tasso, D.; Testi, L.; Vakili, F.; von der Lühe, O.; Valtier, J. -C.; Ventura, N. Bibcode: 2007A&A...464...29T Altcode: 2006astro.ph..3046T Aims:In this paper, we present an innovative data reduction method for single-mode interferometry. It has been specifically developed for the AMBER instrument, the three-beam combiner of the Very Large Telescope Interferometer, but it can be derived for any single-mode interferometer.
Methods: The algorithm is based on a direct modelling of the fringes in the detector plane. As such, it requires a preliminary calibration of the instrument in order to obtain the calibration matrix that builds the linear relationship between the interferogram and the interferometric observable, which is the complex visibility. Once the calibration procedure has been performed, the signal processing appears to be a classical least-square determination of a linear inverse problem. From the estimated complex visibility, we derive the squared visibility, the closure phase, and the spectral differential phase.
Results: The data reduction procedures have been gathered into the so-called amdlib software, now available for the community, and are presented in this paper. Furthermore, each step in this original algorithm is illustrated and discussed from various on-sky observations conducted with the VLTI, with a focus on the control of the data quality and the effective execution of the data reduction procedures. We point out the present limited performances of the instrument due to VLTI instrumental vibrations which are difficult to calibrate.

Based on observations collected

at the European Southern Observatory, Paranal, Chile. Title: Direct constraint on the distance of γ2 Velorum from AMBER/VLTI observations Authors: Millour, F.; Petrov, R. G.; Chesneau, O.; Bonneau, D.; Dessart, L.; Bechet, C.; Tallon-Bosc, I.; Tallon, M.; Thiébaut, E.; Vakili, F.; Malbet, F.; Mourard, D.; Antonelli, P.; Beckmann, U.; Bresson, Y.; Chelli, A.; Dugué, M.; Duvert, G.; Gennari, S.; Glück, L.; Kern, P.; Lagarde, S.; Le Coarer, E.; Lisi, F.; Perraut, K.; Puget, P.; Rantakyrö, F.; Robbe-Dubois, S.; Roussel, A.; Tatulli, E.; Weigelt, G.; Zins, G.; Accardo, M.; Acke, B.; Agabi, K.; Altariba, E.; Arezki, B.; Aristidi, E.; Baffa, C.; Behrend, J.; Blöcker, T.; Bonhomme, S.; Busoni, S.; Cassaing, F.; Clausse, J. -M.; Colin, J.; Connot, C.; Delboulbé, A.; Domiciano de Souza, A.; Driebe, T.; Feautrier, P.; Ferruzzi, D.; Forveille, T.; Fossat, E.; Foy, R.; Fraix-Burnet, D.; Gallardo, A.; Giani, E.; Gil, C.; Glentzlin, A.; Heiden, M.; Heininger, M.; Hernandez Utrera, O.; Hofmann, K. -H.; Kamm, D.; Kiekebusch, M.; Kraus, S.; Le Contel, D.; Le Contel, J. -M.; Lesourd, T.; Lopez, B.; Lopez, M.; Magnard, Y.; Marconi, A.; Mars, G.; Martinot-Lagarde, G.; Mathias, P.; Mège, P.; Monin, J. -L.; Mouillet, D.; Nussbaum, E.; Ohnaka, K.; Pacheco, J.; Perrier, C.; Rabbia, Y.; Rebattu, S.; Reynaud, F.; Richichi, A.; Robini, A.; Sacchettini, M.; Schertl, D.; Schöller, M.; Solscheid, W.; Spang, A.; Stee, P.; Stefanini, P.; Tasso, D.; Testi, L.; von der Lühe, O.; Valtier, J. -C.; Vannier, M.; Ventura, N. Bibcode: 2007A&A...464..107M Altcode: 2006astro.ph.10936M Context: Interferometry can provide spatially resolved observations of massive star binary systems and their colliding winds, which thus far have been studied mostly with spatially unresolved observations.
Aims: We present the first AMBER/VLTI observations, taken at orbital phase 0.32, of the Wolf-Rayet and O (WR+O) star binary system γ2 Velorum and use the interferometric observables to constrain its properties.
Methods: The AMBER/VLTI instrument was used with the telescopes UT2, UT3, and UT4 on baselines ranging from 46 m to 85 m. It delivered spectrally dispersed visibilities, as well as differential and closure phases, with a resolution R=1500 in the spectral band 1.95-2.17 μm. We interpret these data in the context of a binary system with unresolved components, neglecting in a first approximation the wind-wind collision zone flux contribution.
Results: Using WR- and O-star synthetic spectra, we show that the AMBER/VLTI observables result primarily from the contribution of the individual components of the WR+O binary system. We discuss several interpretations of the residuals, and speculate on the detection of an additional continuum component, originating from the free-free emission associated with the wind-wind collision zone (WWCZ), and contributing at most to the observed K-band flux at the 5% level. Based on the accurate spectroscopic orbit and the Hipparcos distance, the expected absolute separation and position angle at the time of observations were 5.1±0.9 mas and 66±15°, respectively. However, using theoretical estimates for the spatial extent of both continuum and line emission from each component, we infer a separation of 3.62+0.11-0.30 mas and a position angle of 73+9-11°, compatible with the expected one. Our analysis thus implies that the binary system lies at a distance of 368+38-13 pc, in agreement with recent spectrophotometric estimates, but significantly larger than the Hipparcos value of 258+41-31 pc.

Based on observations collected at the European Southern Observatory, Paranal, Chile, within the guaranteed time programme 074.A-9025(A). Title: Optical configuration and analysis of the AMBER/VLTI instrument Authors: Robbe-Dubois, S.; Lagarde, S.; Petrov, R. G.; Lisi, F.; Beckmann, U.; Antonelli, P.; Bresson, Y.; Martinot-Lagarde, G.; Roussel, A.; Salinari, P.; Vannier, M.; Chelli, A.; Dugué, M.; Duvert, G.; Gennari, S.; Glück, L.; Kern, P.; Le Coarer, E.; Malbet, F.; Millour, F.; Perraut, K.; Puget, P.; Rantakyrö, F.; Tatulli, E.; Weigelt, G.; Zins, G.; Accardo, M.; Acke, B.; Agabi, K.; Altariba, E.; Arezki, B.; Aristidi, E.; Baffa, C.; Behrend, J.; Blöcker, T.; Bonhomme, S.; Busoni, S.; Cassaing, F.; Clausse, J. -M.; Colin, J.; Connot, C.; Delage, L.; Delboulbé, A.; Domiciano de Souza, A.; Driebe, T.; Feautrier, P.; Ferruzzi, D.; Forveille, T.; Fossat, E.; Foy, R.; Fraix-Burnet, D.; Gallardo, A.; Giani, E.; Gil, C.; Glentzlin, A.; Heiden, M.; Heininger, M.; Hernandez Utrera, O.; Hofmann, K. -H.; Kamm, D.; Kiekebusch, M.; Kraus, S.; Le Contel, D.; Le Contel, J. -M.; Lesourd, T.; Lopez, B.; Lopez, M.; Magnard, Y.; Marconi, A.; Mars, G.; Mathias, P.; Mège, P.; Monin, J. -L.; Mouillet, D.; Mourard, D.; Nussbaum, E.; Ohnaka, K.; Pacheco, J.; Perrier, C.; Rabbia, Y.; Rebattu, S.; Reynaud, F.; Richichi, A.; Robini, A.; Sacchettini, M.; Schertl, D.; Schöller, M.; Solscheid, W.; Spang, A.; Stee, P.; Stefanini, P.; Tallon, M.; Tallon-Bosc, I.; Tasso, D.; Testi, L.; Vakili, F.; von der Lühe, O.; Valtier, J. -C.; Ventura, N. Bibcode: 2007A&A...464...13R Altcode: 2008arXiv0807.3717R Aims:This paper describes the design goals and engineering efforts that led to the realization of AMBER (Astronomical Multi BEam combineR) and to the achievement of its present performance.
Methods: On the basis of the general instrumental concept, AMBER was decomposed into modules whose functions and detailed characteristics are given. Emphasis is put on the spatial filtering system, a key element of the instrument. We established a budget for transmission and contrast degradation through the different modules, and made the detailed optical design. The latter confirmed the overall performance of the instrument and defined the exact implementation of the AMBER optics.
Results: The performance was assessed with laboratory measurements and commissionings at the VLTI, in terms of spectral coverage and resolution, instrumental contrast higher than 0.80, minimum magnitude of 11 in K, absolute visibility accuracy of 1%, and differential phase stability of 10-3 rad over one minute. Title: New high resolution solar telescope GREGOR Authors: Volkmer, R.; von der Lühe, O.; Kneer, F.; Staude, J.; Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvaña, M.; Sobotka, M.; Nicklas, H.; Popow, E.; Puschmann, K. G.; Schmidt, W.; Soltau, D.; Strassmeier, K.; Wittmann, A. D. Bibcode: 2007msfa.conf...39V Altcode: The 1.5m solar telescope GREGOR is being constructed at Tenerife, Spain. Its purpose is to observe with high spatial and spectral resolution small-scale dynamic magnetic features on the Sun. The telescope is completely open with retractable dome and actively cooled primary mirror made of silicon carbide to minimize thermal effects on the image quality. After completion it will be one of the most powerful solar telescopes. This paper presents a general overview of the telescope characteristics and the current status. Title: Advances, challenges and limitations of speckle reconstruction and deconvolution Authors: Mikurda, K.; von der Lühe, O.; Wöger, F.; Schmidt, W. Bibcode: 2007msfa.conf..131M Altcode: This paper presents the experiences with speckle imaging and deconvolution techniques we have made during the last five years at the Kiepenheuer-Institut für Sonnenphysik. We discuss our implementation of the above techniques, their tests and application ranges. In addition, we summarize our efforts in applying speckle techniques to the data taken with the support of the adaptive optics. Title: Anisoplanatic optical transfer functions for solar adaptive optics from simulation using wave front sensor data Authors: Sailer, M.; von der Lühe, O. Bibcode: 2007msfa.conf..115S Altcode: Solar observations with online compensation of atmospheric seeing is possible at the Vacuum Tower Telescope (VTT) on Tenerife (Spain) with the aid of the Kiepenheuer Adaptive Optics (AO) System KAOS. For the post-facto speckle image reconstruction methods used at our institutes, however, problems arise regarding the photometric accuracy of the intensity enhancement at each position of the reconstructed image (Puschmann & Sailer 2006), rooting in the lack of knowledge about the field dependency of the actual residual wave front (WF) after correction and therefore of the instantaneous optical transfer functions (OTFs) at the time of each exposure. To process more adequately particular sets of image data obtained with KAOS, we simulate their OTFs and the according statistically averaged quantity for the reconstruction, the speckle transfer function (STF), with support of an activity log of the AO system during the time of data acquisition. Data regarding the wave front sensor (WFS) and the deformable mirror (DM) are analysed to model the atmospheric conditions and the average compensation power of the AO system. With this information we simulate the propagation of atmospherically distorted WFs through the optical elements of the VTT and KAOS, approaching the average WF error statistics reflected in the experimental data. Title: High Resolution Time Series of Narrowband Ca IIK Images in the Chromosphere Authors: Wöger, F.; Wedemeyer-Böhm, S.; Schmidt, W.; von der Lühe, O. Bibcode: 2006ASPC..354..284W Altcode: We have observed a region of quiet Sun near disk center with the Vacuum Tower Telescope (VTT) of the Kiepenheuer-Institut für Sonnenphysik at the Observatorio del Teide, Tenerife, Spain in April 2005 in several wavelengths. Observations were made at the Ca II K line at 393.3 nm, using a Lyot filter with a bandwidth of 30 ± FWHM, centered at the K_{2v} emission peak; at the Hα line at 656.3 nm, using a Lyot filter (25 ± FWHM) centered at line core, and in the G-band (430.5 nm), using an interference filter (1 nm FWHM). We acquired a two-hour long sequence of images at a cadence of ten seconds and a spatial resolution of about 0.3 arcsec. We present our Ca observations of excellent spatial resolution which show morphological structures in internetwork regions similar in form, size and lifetime to those present in recent numerical models of the solar chromosphere. Title: Basic physical parameters of a selected sample of evolved stars Authors: da Silva, L.; Girardi, L.; Pasquini, L.; Setiawan, J.; von der Lühe, O.; de Medeiros, J. R.; Hatzes, A.; Döllinger, M. P.; Weiss, A. Bibcode: 2006A&A...458..609D Altcode: 2006astro.ph..8160D We present the detailed spectroscopic analysis of 72 evolved stars, which were previously studied for accurate radial velocity variations. Using one Hyades giant and another well studied star as the reference abundance, we determine the [Fe/H] for the whole sample. These metallicities, together with the T_eff values and the absolute V-band magnitude derived from Hipparcos parallaxes, are used to estimate basic stellar parameters (ages, masses, radii, (B-V)0 and log g) using theoretical isochrones and a Bayesian estimation method. The (B-V)0 values so estimated turn out to be in excellent agreement (to within ~0.05 mag) with the observed (B-V), confirming the reliability of the T_eff-(B-V)0 relation used in the isochrones. On the other hand, the estimated log g values are typically 0.2 dex lower than those derived from spectroscopy; this effect has a negligible impact on [Fe/H] determinations. The estimated diameters θ have been compared with limb darkening-corrected ones measured with independent methods, finding an agreement better than 0.3 mas within the 1<θ<10 mas interval (or, alternatively, finding mean differences of just 6%). We derive the age-metallicity relation for the solar neighborhood; for the first time to our knowledge, such a relation has been derived from observations of field giants rather than from open clusters and field dwarfs and subdwarfs. The age-metallicity relation is characterized by close-to-solar metallicities for stars younger than ~4 Gyr, and by a large [Fe/H] spread with a trend towards lower metallicities for higher ages. In disagreement with other studies, we find that the [Fe/H] dispersion of young stars (less than 1 Gyr) is comparable to the observational errors, indicating that stars in the solar neighbourhood are formed from interstellar matter of quite homogeneous chemical composition. The three giants of our sample which have been proposed to host planets are not metal rich; this result is at odds with those for main sequence stars. However, two of these stars have masses much larger than a solar mass so we may be sampling a different stellar population from most radial velocity searches for extrasolar planets. We also confirm the previous indication that the radial velocity variability tends to increase along the RGB, and in particular with the stellar radius. Title: Observation of a short-lived pattern in the solar chromosphere Authors: Wöger, F.; Wedemeyer-Böhm, S.; Schmidt, W.; von der Lühe, O. Bibcode: 2006A&A...459L...9W Altcode: 2006astro.ph..9382W Aims.In this work we investigate the dynamic behavior of inter-network regions of the solar chromosphere.
Methods: .We observed the chromosphere of the quiet Sun using a narrow-band Lyot filter centered at the Ca II K2v emission peak with a bandpass of 0.3 Å. We achieved a spatial resolution of on average 0.7 arcsec at a cadence of 10 s.
Results: .In the inter-network we find a mesh-like pattern that features bright grains at the vertices. The pattern has a typical spatial scale of 1.95 arcsec and a mean evolution time scale of 53 s with a standard deviation of 10 s. A comparison of our results with a recent three-dimensional radiation hydrodynamical model implies that the observed pattern is of chromospheric origin. The measured time scales are not compatible with those of reversed granulation in the photosphere although the appearance is similar. A direct comparison between network and inter-network structure shows that their typical time scales differ by at least a factor of two.
Conclusions: .The existence of a rapidly evolving small-scale pattern in the inter-network regions supports the picture of the lower chromosphere as a highly dynamical and intermittent phenomenon. Title: Asteroseismology of K giants Authors: di Mauro, M. P.; Cardini, D.; D'Antona, F.; Weiss, A.; Döllinger, M. P.; Pasquini, L.; Girardi, L.; Hatzes, A.; von der Lühe, O.; de Medeiros, J. R.; Roth, M.; Setaiwan, J.; da Silva, L. Bibcode: 2006ESASP.624E.118D Altcode: 2006soho...18E.118D No abstract at ADS Title: The new 1.5m solar telescope GREGOR: first light and start of commissioning Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude, Jürgen; Berkefeld, Thomas; Caligari, Peter; Halbgewachs, Clemens; Heidecke, Frank; Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald; Wittmann, Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka, Michal; Klvana, Miroslav; Collados, Manuel Bibcode: 2006SPIE.6267E..0WV Altcode: 2006SPIE.6267E..29V The integration of the three main silicon carbide mirrors into the new 1.5 m solar telescope GREGOR at Izana on Tenerife, Spain is planned during 2006. We expect first light at the end of 2006. A progress report about integration of the optics and mechanics and planning of the commissioning phase of the telescope and post focus instruments will be presented at the meeting. The GREGOR telescope is build by a consortium of the Kiepenheuer Institut fur Sonnenphysik in Freiburg, the Astrophysikalische Institut Potsdam, the Institut fur Astronomie Gottingen and additional national and international Partners. Title: Multi-conjugate solar adaptive optics with the VTT and GREGOR Authors: Berkefeld, Thomas; Soltau, Dirk; von der Lühe, Oskar Bibcode: 2006SPIE.6272E..05B Altcode: 2006SPIE.6272E...4B After the successful demonstration of the solar multi-conjugate adaptive optics (MCAO) system at the German 70cm Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife, in the last years, we are continuing the development of the system as a testbed for the future MCAO of the 150cm GREGOR solar telescope. We describe an improved reconstruction scheme that increases the number of corrected off-axis degrees of freedom and will be tested at the VTT in September 2006. We present a modified optical setup of the GREGOR MCAO that has the advantage of being adjustable to a wide height range of the turbulence. Title: Simulation of adaptive optics for the Vacuum Tower Telescope Authors: Enmark, Anita; Berkefeld, Thomas; von der Lühe, Oskar; Andersen, Torben Bibcode: 2006ExA....21...87E Altcode: 2006ExA...tmp...45E A simulation model of the adaptive optics of the German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife, is presented. The model uses modules from the integrated model of the Euro50 extremely large telescope, and includes submodels of a Shack-Hartmann wavefront sensor, a de-formable mirror, a tip-tilt mirror, high-voltage amplifier low-pass filters, a reconstructor and a controller. We investigate the impact on the closed loop bandwidth of changes in controller configuration and certain system parameters, such as low pass filter bandwidth and camera integration and readout time. Control strategies were tested on simple models before implementation on the full VTT model. Using the models, different control strategies are compared. Title: High resolution solar telescope GREGOR Authors: Peter, H.; von der Luehe, O. Bibcode: 2006cosp...36.3628P Altcode: 2006cosp.meet.3628P GREGOR is a new 1 5 m solar telescope assembled on Tenerife Spain by a German consortium together with international partners It is designed for high-precision measurements of the magnetic field and the gas motion in the solar photosphere and chromosphere with a resolution of 70 km on the Sun and for high resolution stellar spectroscopy The telescope has a Gregorian configuration mounted in an open structure with a retractable dome for thermal control and features a multi-conjugate adaptive optics system GREGOR will be equipped with several post-focus instruments including polarimetric longslit spectrographs for the visible and infrared and a high resolution Fabry-Perot filter spectrometer for 3D spectroscopy Apart from a report on the present status of the instrument the talk will focus on a selection of science goals of GREGOR Among these are the emergence evolution and disappearance of small-scale magnetic flux the energy budget of sunspots and the structure of their penumbrae as well as the dynamics of the chromosphere In this context an attempt will be made to put future observations with GREGOR in the context of modeling of the photosphere chromosphere and corona Title: Why are G and K Giants Radial Velocity Variables? Authors: Doellinger, Michaela Petronilla; Pasquini, Luca; Hatzes, Artie Peter; Setiawan, Johny; da Silva, Licio; Renan de Medeiros, Jose; von der Luehe, Oskar; Girardi, Leo; di Mauro, Maria Pia; Weiss, Achim; Roth, Markus Bibcode: 2005Msngr.122...39D Altcode: During the past year ESO has coordinated a number of instrument concept studies as a complement to the OWL Observatory Design Study. Eight teams of scientists and engineers from different institutes in Europe and ESO have identified a variety of science programmes at the frontier of astrophysics and developed concepts of instruments at OWL which would be able to carry them out. This exercise has provided a first view of the unique astronomical observations at Blue to IR wavelengths which will become possible with a future European Extremely Large Telescope. Title: Multi-conjugate solar adaptive optics at the Vacuum Tower Telescope on Tenerife Authors: von der Lühe, Oskar; Berkefeld, Thomas; Soltau, Dirk Bibcode: 2005CRPhy...6.1139V Altcode: We present a breadboard multi-conjugate adaptive optics (MCAO) system for high angular resolution solar observations which we operate at the Vacuum Tower Telescope. We have developed methods to estimate quantitatively the performance of solar adaptive optics from science data. Several sets of short exposure images of the solar photosphere were analyzed to assess the performance of the MCAO. We demonstrate that a 30 arcsec field of view is substantially improved when the MCAO system is turned on. This compares favourably with an improvement of a 10 arcsec field with conventional solar adaptive optics. We also show how irradiance fluctuations in the MCAO compensated focus can be suppressed. To cite this article: O. von der Lühe et al., C. R. Physique 6 (2005). Title: The new 1.5 solar telescope GREGOR: progress report and results of performance tests Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude, Jürgen; Berkefeld, Thomas; Caligari, Peter; Halbgewachs, Clemens; Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald; Wittmann, Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka, Michal; Klvana, Miroslav; Collados, Manuel Bibcode: 2005SPIE.5901...75V Altcode: The telescope structure including control system and the complete retractable dome of the new 1.5 m solar telescope GREGOR were assembled during 2004 at Izana on Tenerife, Spain. The GREGOR telescope is build by a consortium of the Kiepenheuer Institut fuer Sonnenphysik, the Astrophysikalische Institut Potsdam, the Institut fuer Astrophysik Goettingen and additional national and international Partners. Pointing, tracking and thermal tests were made to verify the proposed performance. The results of these tests and a progress report of the project will be presented. Title: Results of the multi-conjugate adaptive optics system at the German solar telescope, Tenerife Authors: Berkefeld, Thomas; Soltau, Dirk; von der Luehe, Oskar Bibcode: 2005SPIE.5903..219B Altcode: We present the optical setup, reconstruction scheme and observational results of the Multi-conjugate Adaptive Optics (MCAO) system at the German 70cm Vacuum Tower Telescope, Observatorio del Teide, Tenerife. The system serves as a testbed for the future MCAO of the new 1.5m GREGOR solar telescope and is an extension of the conventional Adaptive Optics (CAO) system. We demonstrate that the use of one additional MCAO wavefront sensor and one additional deformable mirror increases the corrected field of view from 10 to 35 arcseconds. Title: A substellar companion around the intermediate-mass giant star HD 11977 Authors: Setiawan, J.; Rodmann, J.; da Silva, L.; Hatzes, A. P.; Pasquini, L.; von der Lühe, O.; de Medeiros, J. R.; Döllinger, M. P.; Girardi, L. Bibcode: 2005A&A...437L..31S Altcode: 2005astro.ph..5510S We report the discovery of a substellar companion to the intermediate-mass star HD 11977 (G5 III). Radial velocities of this star have been monitored for five years with FEROS at the 1.52-m ESO and later at the 2.2-m MPG/ESO telescope in La Silla, Chile. Based on the collected data we calculated an orbital solution with a period of P=711 days, a semi-amplitude of K1=105 m s-1, and an eccentricity of e=0.4. The period of the radial-velocity variation is longer than that of the estimated stellar rotation, rendering it unlikely that rotational modulation is the source of the variation in the radial velocity. This hypothesis is supported by the absence of a correlation between stellar activity indicators and radial-velocity variation. By determining a primary stellar mass of Mstar=1.91 M⊙, the best-fit minimum mass of the companion and semi-major axis of the orbit are m2sin{i}=6.54 MJup and a2=1.93 AU, respectively. An upper limit for the mass of the companion of m2 ⪉ 65.5 MJup has been calculated from Hipparcos astrometric measurements. Although the possibility of a brown-dwarf companion cannot be excluded, HD 11977 B is one of the few planet candidates detected around an intermediate-mass star. The progenitor main-sequence star of HD 11977 is probably an A-type star. This discovery gives an indirect evidence for planetary companions around early type main-sequence stars. Title: Chromospheric activity of red giants Authors: Setiawan, J.; von der Lühe, O.; Pasquini, L.; da Silva, L.; Hatzes, A.; Klotz, F.; Girardi, L.; de Medeiros, J. R. Bibcode: 2005ESASP.560..963S Altcode: 2005csss...13..963S No abstract at ADS Title: Interferometry Authors: von der Lühe, Oskar Bibcode: 2005ASIB..198..275V Altcode: 2005opas.conf..275V No abstract at ADS Title: Binaries from FEROS radial velocity survey Authors: Setiawan, J.; da Silva, L.; Pasquini, L.; Hatzes, A. P.; von der Luhe, O.; Girardi, L.; Guenther, E. Bibcode: 2004ASPC..318..283S Altcode: 2004ssrc.conf..283S At least eleven binary systems have been found from our radial velocity (RV) survey (precision ∼ 25 m s-1) of eighty-three G and K evolved stars using FEROS at the 1.52m ESO in La Silla and high resolution echelle-spectrograph at 2m-Alfred-Jensch- telescope in Tautenburg, Germany. We used the simultaneous calibration method and the iodine absorption-cell technique to measure the RV variations. Orbital elements have been derived from the RV curves of six binaries. The stellar companions have minimum masses (m

sin{i}) from 0.1 up to 0.6 Msun and orbital semi-major axes between 0.2 and 3.6 AU. Two special cases are found among them: first, in one system, HD 156111, the sin{i} has been determined which permitted us to derive the companion's real mass, and second, we found a dependence between the RV variation of HD 62644 induced by its companion, and chromospheric activity of the primary star. For the remaining five stars we do not have enough measurements to obtain the complete orbital solutions. Finally, we report two other giants in our sample, HD 224533 and HD 27256, which might have brown-dwarf companions. Title: Adaptive optics for robotic telescopes Authors: von der Lühe, O. Bibcode: 2004AN....325..613V Altcode: Adaptive optics is a technology which compensates dynamical optical aberrations in an astronomical telescope. The technique has matured during the past decade and exists today at any large observatory. Adaptive optics improve the spatial resolution, ideally to the diffraction limit, and concentrate the source's flux, increasing the sensitivity. Both properties can be relevant to science programs with robotic telescopes. The hardware for adaptive optics becomes more readily available at lower costs, and there are many research groups working on their development. We review the current status of adaptive optics and discuss its relevance to robotic telescopes. Title: Second-generation adaptive optics for the 1.5 m solar telescope GREGOR, Tenerife Authors: Berkefeld, Thomas; Soltau, Dirk; von der Luhe, Oskar F. H. Bibcode: 2004SPIE.5490..260B Altcode: We present the optical setup and properties of the second-generation adaptive optics (AO) for the 1.5 m solar telescope GREGOR. The system will consist of a high order AO system correcting about 200 degrees of freedom on-axis at a bandwith of 200 Hz and a multi-conjugate (MCAO) extension that uses one additional deformable mirror to correct the low-order aberrations across a field of one arcminute at a bandwidth of 50 Hz. Diffraction limited observations will be possible for seeing better than 1.2 arcsec. First light is expected in 2007. Title: High-precision wavefront sensor for the SUNRISE Telescope Authors: Schmidt, Wolfgang; Berkefeld, Thomas; Friedlein, Ruediger; Heidecke, Frank; Kentischer, Thomas; von der Lühe, Oskar F.; Sigwarth, Michael; Soltau, Dirk; Walde, E. Bibcode: 2004SPIE.5489.1164S Altcode: SUNRISE is a 1m solar telescope for the visible and near UV wavelength range. It will be flown in long duration stratospheric balloon flights in Antarctica, with a first scientific flight in 2007. In this paper, we describe the development of a wave-front sensing system that will be used for the automatic in-flight alignment of the SUNRISE telescope and for high-precision tracking. The system is based on the principles of an adaptive optics system. A 19-element wavefront sensor is used to determine low order aberrations of the telescope, including defocus and spherical aberrations. The correction is achieved by controlling the position of the telescope secondary and a focusing mirror in closed-loop. In addition to these quasi-static aberrations, the system will also measure image motion with a dynamical range of at least 30 Hz and with a precision of about 0.005 arcs. To this end, the image displacement measured in all sub-apertures is averaged and used as tip-tilt correction signal. This signal will feed a second closed-loop system that drives the tip-tilt mirror assembly. The tip-tilt mirror unit is designed as a dual-stage system that consists of a slow component with a large range of 60 arcs and a fast component with high bandwidth. Title: Progress report of the 1.5 m solar telescope GREGOR Authors: Volkmer, Reiner; von der Lühe, Oskar F.; Kneer, Franz; Staude, Jürgen; Berkefeld, Thomas; Caligari, Peter; Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald; Wiehr, Eberhardt; Wittmann, Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka, Michal; Klvana, Miroslav; Collados, Manuel Bibcode: 2004SPIE.5489..693V Altcode: GREGOR is the new 1.5 m solar telescope assembled on Tenerife, Spain, by the German consortium of the Kiepenheuer Institut fur Sonnenphysik, the Astronomischen Institut Potsdam, the Universitats-Sternwarte Gottingen and other national and international Partners. The refurbishment of the building is almost finished. The manufacturing of the telescope structure and the optics is still in progress. After the integration of the new complete retractable dome in July 2004 the telescope structure, optic and post focus instruments will be assembled during the rest of the year. First light is planned during May 2005. Title: GISOT: a giant solar telescope Authors: Hammerschlag, Robert H.; von der Lühe, Oskar F.; Bettonvil, Felix C.; Jägers, Aswin P.; Snik, Frans Bibcode: 2004SPIE.5489..491H Altcode: A concept is presented for an extremely large high-resolution solar telescope with an aperture of 11 m and diffraction limited for visual wavelengths. The structure of GISOT will be transparent to wind and placed on a transparent stiff tower. For efficient wind flushing, all optics, including the primary mirror, will be located above the elevation axis. The aperture will be of the order of 11 m, not rotatively symmetrical, but of an elongated shape with dimensions 11 x 4 m. It consists of a central on-axis 4 m mirror with on both sides 3 pieces of 2 m mirrors. The optical layout will be kept simple to guarantee quality and minimize stray light. A Coudé room for instruments is planned below the telescope. The telescope will not be housed in a dome-like construction, which interferes with the open principle. Instead the telescope will be protected by a foldable tent construction with a diameter of the order of 30 m, which doesn"t form any obstruction during observations, but can withstand the severe weather circumstances on mountain sites. Because of the nature of the solar scene, extremely high resolution in only one dimension is sufficient to solve many exciting problems in solar physics and in this respect the concept of GISOT is very promising. Title: Photometric stability of multiconjugate adaptive optics Authors: von der Luhe, Oskar F. H. Bibcode: 2004SPIE.5490..617V Altcode: We observe that a multi-conjugate adaptive optics (MCAO) system produces rapid fluctuations of irradiance of a few percent in the compensated focal plane when observing an extended target, such as the solar surface. Such fluctuations are absent in the uncompensated focal plane. The fluctuations are apparently correlated with the local curvature of deformable mirror in the plane conjugated to the high altitude turbulence layer. They can be explained by the field dependent change of effective focal length introduced by a high altitude turbulence layer, which is related to image distortion. Using a simple geometric model of the MCAO system, we are able to reproduce the observed irradiance effects. We propose to introduce a slightly undersized aperture stop at the location of the nominal exit pupil in order to remove the fluctuations in the compensated field. Title: Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI Authors: Leinert, Ch.; van Boekel, R.; Waters, L. B. F. M.; Chesneau, O.; Malbet, F.; Köhler, R.; Jaffe, W.; Ratzka, Th.; Dutrey, A.; Preibisch, Th.; Graser, U.; Bakker, E.; Chagnon, G.; Cotton, W. D.; Dominik, C.; Dullemond, C. P.; Glazenborg-Kluttig, A. W.; Glindemann, A.; Henning, Th.; Hofmann, K. -H.; de Jong, J.; Lenzen, R.; Ligori, S.; Lopez, B.; Meisner, J.; Morel, S.; Paresce, F.; Pel, J. -W.; Percheron, I.; Perrin, G.; Przygodda, F.; Richichi, A.; Schöller, M.; Schuller, P.; Stecklum, B.; van den Ancker, M. E.; von der Lühe, O.; Weigelt, G. Bibcode: 2004A&A...423..537L Altcode: We present the first long baseline mid-infrared interferometric observations of the circumstellar disks surrounding Herbig Ae/Be stars. The observations were obtained using the mid-infrared interferometric instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI on Cerro Paranal. The 102 m baseline given by the telescopes UT1 and UT3 was employed, which provides a maximum full spatial resolution of 20 milli-arcsec (mas) at a wavelength of 10 μm. The interferometric signal was spectrally dispersed at a resolution of 30, giving spectrally resolved visibility information from 8 μm to 13.5 μm. We observed seven nearby Herbig Ae/Be stars and resolved all objects. The warm dust disk of HD 100546 could even be resolved in single-telescope imaging. Characteristic dimensions of the emitting regions at 10 μm are found to be from 1 AU to 10 AU. The 10 μm sizes of our sample stars correlate with the slope of the 10-25 μm infrared spectrum in the sense that the reddest objects are the largest ones. Such a correlation would be consistent with a different geometry in terms of flaring or flat (self-shadowed) disks for sources with strong or moderate mid-infrared excess, respectively. We compare the observed spectrally resolved visibilities with predictions based on existing models of passive centrally irradiated hydrostatic disks made to fit the SEDs of the observed stars. We find broad qualitative agreement of the spectral shape of visibilities corresponding to these models with our observations. Quantitatively, there are discrepancies that show the need for a next step in modelling of circumstellar disks, satisfying both the spatial constraints such as are now available from the MIDI observations and the flux constraints from the SEDs in a consistent way.

Based on observations made with the Very Large Telescope Interferometer at Paranal Observatory. Title: Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch Authors: Setiawan, J.; Pasquini, L.; da Silva, L.; Hatzes, A. P.; von der Lühe, O.; Girardi, L.; de Medeiros, J. R.; Guenther, E. Bibcode: 2004A&A...421..241S Altcode: We present the results of our radial velocity (RV) measurements of G and K giants, concentrating on the presence of multiple systems in our sample. Eighty-three giants have been observed for 2.5 years with the fiber-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in La Silla, Chile. Seventy-seven stars (93%) of the targets have been analyzed for RV variability using simultaneous Th-Ar calibration and a cross-correlation technique. We estimate the long-term precision of our measurement as better than 25 m s-1. Projected rotational velocities have been measured for most stars of the sample. Within our time-base only 21 stars (or 27%) show variability below 2\sigma, while the others show RV variability with amplitudes up to several km s-1. The large amplitude (several km s-1) and shape (high eccentricity) of the RV variations for 11 of the program stars are consistent with stellar companions, and possibly brown dwarf companions for two of the program stars. In those systems for which a full orbit could be derived, the companions have minimum masses from ∼0.6 M\sun down to 0.1 M\sun. To these multiple systems we add the two candidates of giant planets already discovered in the sample. This analysis shows that multiple systems contribute substantially to the long-term RV variability of giant stars, with about 20% of the sample being composed of multiple systems despite screening our sample for known binary stars. After removing binaries, the range of RV variability in the whole sample clearly decreases, but the remaining stars retain a statistical trend of RV variability with luminosity: luminous cool giants with B-V≥1.2 show RV variations with \sigma_{/lineRV} > 60 m s-1, while giants with B-V < 1.2 including those in the clump region exhibit less variability or they are constant within our accuracy. The same trend is observed with respect to absolute visual magnitudes: brighter stars show a larger degree of variability and, when plotted in the RV variability vs. magnitude diagram a trend of increasing RV scatter with luminosity is seen. The amplitude of RV variability does not increase dramatically, as predicted, for instance, by simple scaling laws. At least two luminous and cooler stars of the sample show a correlation between RV and chromospheric activity and bisector asymmetry, indicating that in these two objects RV variability is likely induced by the presence of (chromospheric) surface structures.

Based on observations collected at the 1.52 m-ESO telescope at the La Silla Observatory from Oct 1999 to Feb. 2002 under ESO programs and the ESO-Observatório Nacional, Brazil, agreement and in part on observations collected on the Alfred Jensch 2 m telescope of the Thüringer Landessternwarte Tautenburg. Title: Adaptive optics transfer function estimation for solar observations. Authors: Sailer, M.; von der Luehe, O.; Kneer, F. Bibcode: 2004ANS...325..130S Altcode: 2004ANS...325..P65S; 2004ANS...325a.130S No abstract at ADS Title: Optical and thermal design of the main optic of the solar telescope GREGOR Authors: Volkmer, Reiner; von der Luhe, Oskar; Soltau, Dirk; Emde, Peter; Krodel, Matthias; Pailer, Norbert; Wiehr, Eberhardt Bibcode: 2003SPIE.5179..270V Altcode: The optical and thermal design of the 1.5 m solar telescope GREGOR is presented. The three first main mirrors of GREGOR will be made from Cesic, a silicon carbide material. One major constraint of large solar telescopes is the thermal load of the structure and the mirrors. The mirrors are heated by the solar radiation and introduce potentially harmful mirror seeing. GREGOR will use an active mirror cooling system and an open telescope structure to reduce these negative effects. A thermal analysis shows that the equilibrium temperature of the Cesic Mirror without active cooling is 6° above ambient temperature. Additional cooling will reduce the temperature difference of the optical surface and ambient air to below 0.1° K. With tempered airflow (about 2.5 m3/s per square meter mirror surface) the temperature gradient on the surface of the face sheet is less than 0.1°K. The telescope will have an open structure and a complete retractable dome to support mirror and structure cooling by wind. Title: Sensitivity of Active and Passive High Resolution Techniques Authors: von der Lühe, Oskar Bibcode: 2003ANS...324...23V Altcode: 2003ANS...324..C05V No abstract at ADS Title: Current Status of the 1.5m Solar Telescope GREGOR Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude, Jürgen; Berkfeld, Thomas; Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald; Wiehr, Eberhardt; Wittman, Axel; Hofmann, Axel; Sobotka, Michal; Klvana, Miroslav Bibcode: 2003ANS...324..112V Altcode: 2003ANS...324..P19V No abstract at ADS Title: Solar Imaging with an Extended Knox-Thompson Technique Authors: Mikurda, K.; von der Lühe, O.; Wöger, F. Bibcode: 2003ANS...324..112M Altcode: 2003ANS...324..P18M No abstract at ADS Title: Transfer Function Calibration for Speckle Reconstruction Authors: Sailer, Markus; von der Lühe, Oskar; Kneer, Franz Bibcode: 2003ANS...324...23S Altcode: 2003ANS...324..C04S No abstract at ADS Title: Dynamics of the G-band Bright Points Authors: Mikurda, K.; von der Lühe, O.; Schmidt, W. Bibcode: 2003ANS...324...24M Altcode: 2003ANS...324..C06M No abstract at ADS Title: GREGOR: the new 1.5m solar telescope on Tenerife Authors: Volkmer, Reiner; von der Luehe, Oskar; Kneer, Franz; Staude, Juergen; Hofmann, Axel; Schmidt, Wolfgang; Sobotka, Michal; Soltau, Dirk; Wiehr, Eberhardt; Wittmann, Axel; Berkefeld, Thomas Bibcode: 2003SPIE.4853..360V Altcode: The new 1.5 m high resolution telescope will be build up on the reused solar tower of the German 45 cm Gregory Coudé Telescope at the Teide Observatory, Izaña, Tenerife. The new telescope is a Gregory type with open telescope structure, alt-azimuth mount, complete retractable dome, and a pool of well established and new developed post focus instruments. An adaptive optics system provides the capability for diffraction limited observations at visible wavelengths and the polarimetry device in the secondary focus reduces the perturbation due to instrumental polarization in an efficient way. We describe the main optical characteristics and the focal plane instrumentation with respect to the latest status of the project. Title: Evidence of a sub-stellar companion around HD 47536 Authors: Setiawan, J.; Hatzes, A. P.; von der Lühe, O.; Pasquini, L.; Naef, D.; da Silva, L.; Udry, S.; Queloz, D.; Girardi, L. Bibcode: 2003A&A...398L..19S Altcode: We report evidence of a low-mass companion around the K1III giant star HD 47536. This star belongs to our sample of 83 subgiant and giant stars studied for their radial velocity variations using the FEROS spectrograph at the 1.52 m-ESO telescope on La Silla. We find that the radial velocity of HD 47536 exhibits a periodic variation of about 712 days with a semi-amplitude of 113 m s-1. These variations are not accompanied by variations in either Ca II emission or in the spectral line shapes. A Keplerian orbit due to a sub-stellar companion is thus the most viable explanation for the radial velocity variation. Assuming a moderate stellar mass of m1= 1.1-3.0 Msun we obtain a minimum mass for the companion of m2 sin i= 5.0-9.7 MJup, an orbital semi-major axis of 1.6-2.3 AU, and an eccentricity of e=0.2.

Based on observations collected at the ESO 1.52m telescope at the La Silla Observatory from July 2000 to February 2002 under ESO programs and the ESO-Observatório Nacional, Brazil, agreement. Title: Lightweight Cesic mirrors and their applications Authors: Pailer, Norbert M.; Kroedel, Matthias R.; Rosenberg, William J.; Schmidt, Wolfgang; von der Luehe, Oskar Bibcode: 2003SPIE.4853..427P Altcode: Ceramics technologies were successfully applied to a series of lightweight mirrors with different sizes and requirements. Several joining and optical surface optimizations were applied. Besides the classical rib-structured mirrors also the application of sandwich mirrors with Cesic foam and/or honeycomb structures are going to be tested. For all processes relatively simple straightforward processes can be applied which keeps the products relatively cost-effective. Title: Multi-conjugate Adaptive Optics at the Vacuum Tower Telescope, Tenerife Authors: Berkefeld, Thomas; Soltau, Dirk; von der Lühe, Oskar Bibcode: 2003SPIE.4839..544B Altcode: We present an overview of the optical setup and control algorithms for the multi-conjugate adaptive optics (MCAO) system of the 70cm German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife. The system is designed to remove the strong differential tip/tilt of the present AO system across a field of 30 arcseconds at visible wavelengths. It will consist of two Shack-Hartmann wavefront sensors (WFS) and two deformable mirrors (DM) plus a separate Tip/Tilt mirror. Both wavefront sensors will be situated in the pupil plane of the telescope. One determines the high order wavefront aberrations for the center of the field of view (FOV), the other measures only low order wavefront aberrations, but covers a large FOV in each subaperture. A 35 actuator bimorph mirror and a 37 actuator membrane mirror will correct the ground layer and the tropopause, respectively. For wavefront reconstruction, the mirror eigenmodes will be used. The system will have first light in the first quarter of 2003. Scientific operation is expected to start in April 2003 or July 2003. Title: GREGOR - optical design considerations Authors: Soltau, D.; Berkefeld, Th.; von der Lühe, O.; Hofmann, A.; Schmidt, W.; Volkmer, R.; Wiehr, E. Bibcode: 2003AN....324..292S Altcode: After many years of successful work the Gregory Coudé Telescope (GCT) is going to be replaced by GREGOR. This new 1.5 m telescope is the result of design considerations which to a wide extent are based on new technologies. Special aspects of the design are presented, including measures to compensate for optical aberrations caused by instrumental and atmospheric effects (Adaptive Optics). First light is expected for the end of 2004. Title: MIDI - the 10 \mum instrument on the VLTI Authors: Leinert, Ch.; Graser, U.; Przygodda, F.; Waters, L. B. F. M.; Perrin, G.; Jaffe, W.; Lopez, B.; Bakker, E. J.; Böhm, A.; Chesneau, O.; Cotton, W. D.; Damstra, S.; de Jong, J.; Glazenborg-Kluttig, A. W.; Grimm, B.; Hanenburg, H.; Laun, W.; Lenzen, R.; Ligori, S.; Mathar, R. J.; Meisner, J.; Morel, S.; Morr, W.; Neumann, U.; Pel, J. -W.; Schuller, P.; Rohloff, R. -R.; Stecklum, B.; Storz, C.; von der Lühe, O.; Wagner, K. Bibcode: 2003Ap&SS.286...73L Altcode: After more than five years of preparation, the mid-infrared interferometric instrument MIDI has been transported to Paranal where it will undergo testing and commissioning on the Very Large Telescope Interferometer VLTI from the end of 2002 through large part of this year 2003. Thereafter it will be available as a user instrument to perform interferometric observations over the 8 μm-13 μm wavelength range, with a spatial resolution of typically 20 milliarcsec, a spectral resolution of up to 250, and an anticipated point source sensitivity of N = 3-4 mag or 1-2.5 Jy for self - fringe tracking, which will be the only observing mode during the first months of operation. We describe the layout of the instrument, laboratory tests, and expected performance, both for broadband and spectrally resolved observing modes. We also briefly outline the planned guaranteed time observations. Title: Multi-conjugate adaptive optics for the 1.5 m GREGOR telescope Authors: Berkefeld, T.; Soltau, D.; von der Lühe, O. Bibcode: 2003AN....324..296B Altcode: No abstract at ADS Title: Precise radial velocity measurements of G and K giants. First results Authors: Setiawan, J.; Pasquini, L.; da Silva, L.; von der Lühe, O.; Hatzes, A. Bibcode: 2003A&A...397.1151S Altcode: We present the first results of our precise radial velocity (RV) measurements of G and K giants. A number of stars from our list of 80 targets have been observed for 14 months using the fibre-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in La Silla, Chile. This sample increases the number of giants surveyed with precise stellar radial velocity measurements at least by a factor of 10. During this period we are able to estimate the long-term accuracy of our measurement as better than 11 m s-1. We use the simultaneous Th-Ar calibration and cross-correlation technique to compute the radial velocity by applying a numerical template for K-type stars. Standard deviation sigma of mean radial velocity variations between 3 m s-1 and 4 km s-1 with timescales between several days and years are measured for 21 of G and K giants which are presented in this paper. Fifteen stars show definite variability above 3 sigma of our measurement uncertainties. Two stars with RV variations above 800 m s-1 are tentatively identified as new binaries. Although definitive trends between RV variations and stellar evolutionary status cannot yet be established, all the luminous cool giants of our sample seem to have significant radial velocity variations, while those stars in the giant's clump region can be either variable or constant.

Based on observations collected at the ESO 1.52 m telescope at the La Silla Observatory under program ESO No. 64.L-0047, 65.L-0571, 66.D-0592 and from Nov. 99 to Feb. 01 under the ESO-Observatório Nacional, Brazil, agreement.} Title: GREGOR, a 1.5 m Gregory-type telescope for solar observation Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Soltau, W. Schmidt D.; Staude, J.; Wiehr, E.; Wittmann, A. D. Bibcode: 2002NCimC..25..689K Altcode: No abstract at ADS Title: Adaptive optics and multi-conjugate adaptive optics with the VTT Authors: Soltau, D.; Berkefeld, Th.; von der Lühe, O.; Wöger, F.; Schelenz, Th. Bibcode: 2002AN....323..236S Altcode: We are currently developing adaptive optics (AO) system with a multi-conjugate extension for the German solar vacuum tower telescope (VTT) at the Teide Observatory on Tenerife. Multi-conjugate adaptive optics (MCAO) is a technique for increasing the field of view by compensating atmospheric turbulence along several, adjacent lines of sight. A conventional AO system compensates only a single line of sight in the direction of the lock point of its wavefront sensor. At larger field angles, the light from the source transverses higher layers of turbulence which are not sampled by the conventional system. Measurements at the VTT indicate that full compensation is typically restricted to a field of about 10 arcsec in diameter at visible wavelengths. An MCAO uses (at least) a second deformable mirror close to the focal plane of the telescope to compensate a larger field. The sun is a privileged target for an MCAO because the wavefront errors at larger field angles are easily measured. We intend to extend our existing AO system with a second deformable mirror and a second wavefront sensor which enables us to extend the compensated field by a factor of three in diameter. We present and discuss our concept. Title: Measuring starspots on magnetically active stars with the VLTI Authors: Wittkowski, M.; Schöller, M.; Hubrig, S.; Posselt, B.; von der Lühe, O. Bibcode: 2002AN....323..241W Altcode: 2002astro.ph..6090W We present feasibility studies to directly image stellar surface features, which are caused by magnetic activity, with the Very Large Telescope Interferometer (VLTI). We concentrate on late type magnetically active stars, for which the distribution of starspots on the surface has been inferred from photometric and spectroscopic imaging analysis. The study of the surface spot evolution during consecutive rotation cycles will allow first direct measurements (apart from the Sun) of differential rotation which is the central ingredient of magnetic dynamo processes. The VLTI will provide baselines of up to 200 m, and two scientific instruments for interferometric studies at near- and mid-infrared wavelengths. Imaging capabilities will be made possible by closure-phase techniques. We conclude that a realistically modeled cool surface spot can be detected on stars with angular diameters exceeding ~ 2 mas using the VLTI with the first generation instrument AMBER. The spot parameters can then be derived with reasonable accuracy. We discuss that the lack of knowledge of magnetically active stars of the required angular size, especially in the southern hemisphere, is a current limitation for VLTI observations of these surface features. Title: Multiconjugate adaptive optics at the Vacuum Tower Telescope, Tenerife Authors: Berkefeld, Thomas; Soltau, Dirk; von der Luehe, Oskar Bibcode: 2002SPIE.4538..119B Altcode: We present the optical setup and wavefront reconstruction algorithms for the multi-conjugate adaptive optics (MCAO) system at the 70 cm German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife. The system is designed to increase the corrected field of view (FOV) from about 10 arcseconds to 30 arcseconds in the visible. It will consist of two Shack-Hartmann wavefront sensors (WFS) and two deformable mirrors (DMs). Both wavefront sensors will be situated in the pupil plane of the telescope. One determines the high order wavefront aberrations for the center of the FOV, the other measures only low order wavefront aberrations, but covers a large FOV in each subaperture. A 35 actuator bimorph mirror and a micro mirror will correct the ground layer and the tropopause, respectively. The system will have first light in early 2002. Scientific operation is expected to start in the second half of 2002. Title: Adaptive optics system for a 1.5-m solar telescope Authors: von der Luehe, Oskar; Berkefeld, Thomas; Soltau, Dirk Bibcode: 2002SPIE.4538..197V Altcode: We describe an adaptive optics system for the 1.5m diameter solar telescope GREGOR which is currently developed for the Teide Observatory on Tenerife. In a first development step, the AO will provide compensation of 77 modes of wavefront deformation, corresponding to the first 10 radial degrees of a Karhunen-Loeve decomposition of wavefront error. We estimate that such a performance will render GREGOR nearly diffraction limited at visible wavelengths in conditions which correspond to the best 25% of the seeing In Tenerife. The AO uses a Shack-Hartmann wavefront sensor which operates on fine structure anywhere on the solar disk. The required control bandwidth will be about 200 Hz. We show how such a system can be realized using existing technology. Substantial increases in performance in terms of corrected wavefront error and field requires significant technological advances, in particular in the field of large field high speed detectors. Title: GREGOR: a 1.5 m telescope for solar research Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Berkefeld, Th.; Kneer, F.; Staude, J. Bibcode: 2001AN....322..353V Altcode: GREGOR will be a new 1.5 meter solar telescope at the Teide Observatory, Izaña, Tenerife. It will provide observations with high polarimetric precision at visible and infra-red wavelengths from Earth in the solar photosphere and chromosphere with a resolution of 70 km on the Sun. GREGOR replaces the 45 cm Gregory Coudé telescope. Title: Post-focus instrumentation for GREGOR Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.; Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D. Bibcode: 2001AN....322..361K Altcode: GREGOR is a high-resolution solar telescope with an aperture of 1.5 m. It will be equipped with an Adaptive Optics system and is designed for high-precision measurements of magnetic fields and plasma motions in the solar photosphere and chromosphere with a resolution of 70 km on the Sun. GREGOR will replace the Gregory Coudé Telescope at the Observatorio del Teide on Tenerife. In concert with the other solar telescopes at Teide Observatory it will be useful for studying the dynamics of the solar atmosphere and the underlying physical processes. GREGOR will also serve as a test bed for next generation solar telescopes. We discuss briefly the postfocus instrumentation of GREGOR. Title: Visible Imager / Magnetograph: summary of splinter session Authors: von der Lühe, O.; Fleck, B. Bibcode: 2001ESASP.493..149V Altcode: 2001sefs.work..149V Both splinter sessions for the Visible Imager and Magnetograph (VIM) were well attended by approximately 30 participants. The goals of the session was to review the scientific objectives for VIM, to discuss specific technological issues, and to refine the approach to science operation during all orbital phases. The session was closed by the formation of a definition team which will look in detail into these matters in order to prepare a VIM Phase A proposal. Title: GREGOR: high resolution solar observations from 1 AU Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Kneer, F.; Staude, J.; Pailer, N. Bibcode: 2001ESASP.493..417V Altcode: 2001sefs.work..417V No abstract at ADS Title: A proposal for the visible-light imager magnetograph Authors: von der Lühe, O.; Martinez Pillet, V.; Schmidt, W.; Collados, M. Bibcode: 2001ESASP.493..421V Altcode: 2001sefs.work..421V No abstract at ADS Title: Solar Instrumentation. An Introduction Authors: von der Lühe, O. Bibcode: 2001ASSL..259...43V Altcode: 2001dysu.conf...43V Solar instrumentation is a vast field -- it embraces all experimental means to research the Sun. At an observatory -- the most comprehensive entity -- several facilities may be operated. Each facility includes a telescope, sometimes a specialized telescope, which is equipped with one or several post-focus instruments, like spectrographs and filtergraphs, and detectors. The equipment is controlled by specific hardware and software. The collected data is analyzed using equally specialized software packages. An observatory may be ground based or space based. All have in common that their prime use is for solar research; research on nighttime sources is rarely done with these instruments. Most of the time, the equipment collects and detects electromagnetic radiation throughout its entire range from gamma rays to the radio regime. There are also facilities which detect other particles; e.g., the Neutrino observatories or in situ solar wind analysis instruments on spacecraft. In this lecture we shall confine ourselves only to ground based solar instrumentation related to electromagnetic field detection. Instrumentation plays a prominent role in the process of physical understanding. The observation is the essence of experimental solar physics. What can be observed with which precision and accuracy depends mainly on instrumental capabilities, which are driven by technological innovations outside the area of solar physics and even astrophysics. A good example are modern, moderately priced solid state detectors which would have been impossible without their use in consumer electronics. The interpretation of the results from observations leads to new insights in the form of improved physical models of the Sun, which in turn stimulated better understanding of physics as a whole. Title: Prospects of the Research on Cool Stars with Very Large Telescopes and New Developments in Instrumentation (CD-ROM Directory: contribs/pasquini) Authors: Pasquini, L.; Hatzes, A.; von der Lühe, O.; Wiedemann, G. Bibcode: 2001ASPC..223..423P Altcode: 2001csss...11..423P No abstract at ADS Title: GREGOR, a 1.5 m Gregory-type Telescope for Solar Observation Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.; Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D. Bibcode: 2001AGM....18.P223K Altcode: GREGOR is a high-resolution solar telescope with an aperture of 1.5 m. It will be equipped with an Adaptive Optics system and is designed for high-precision measurements of magnetic fields and plasma motions in the solar atmosphere and chromosphere with a resolution of 70 km on the Sun. GREGOR will replace the Gregory Coudé Teleskope at the Observatorio del Teide on Tenerife. We describe the optical design and the focal plane instrumentation. In concert with the other solar telescopes at Teide Observatory it will be useful for studying the dynamics of the solar atmosphere and the underlying processes. GREGOR will also serve as a test bed for next generation solar telescopes. Title: Photospheric Structure from High Resolution Observations (CD-ROM Directory: contribs/luhe) Authors: von der Lühe, O. Bibcode: 2001ASPC..223..153V Altcode: 2001csss...11..153V No abstract at ADS Title: Where is the Intrinsic Size Limit of Fine Structure in the Solar Photosphere? (CD-ROM Directory: contribs/bruls) Authors: Bruls, J. H. M. J.; von der Lühe, O. Bibcode: 2001ASPC..223..589B Altcode: 2001csss...11..589B No abstract at ADS Title: A study of the activity of G and K giants through their precise radial velocity. Breaking the 10-m/sec accuracy with FEROS. Authors: Setiawan, J.; Pasquini, L.; da Silva, L.; Hatzes, A.; von der Luhe, O.; Kaufer, A.; Girardi, L.; de La, Reza R.; de Medeiros, J. R. Bibcode: 2000Msngr.102...13S Altcode: Asteroseismology is an indispensable tool that uses the properties of stellar oscillations to probe the internal structure of stars. This can provide a direct test of stellar structure and evolution theory. Precise stellar radial velocity (RV) measurements made in recent years have not only discovered the first extra-solar planets, but have also uncovered new classes of low-amplitude variable stars. One such is represented by the K giant stars which exhibit RV variations with amplitudes in the range of 50-300 m/s (Walker et al. 1989, Hatzes & Cochran 1993,1994 ab). This variability is multi-periodic and occurring on two time- scales: less than 10 days and several hundreds of days. Title: High spatial resolution performance of a triple Fabry-Pérot filtergraph Authors: von der Lühe, O.; Kentischer, Th. J. Bibcode: 2000A&AS..146..499V Altcode: We investigate the consequences of monochromatic pupil apodisation in a filtergraph based on Fabry-Pérot etalons in a telecentric mount, an effect which was discovered by Beckers (\cite{beckers1}). The pupil apodisation gives rise to systematic changes of the point spread function in the presence of strong spectral gradients, like in the wings of a Fraunhofer line. These changes are likely to produce spurious velocity signals in Dopplergrams generated from high spatial resolution filtergrams of the solar surface. We include in our analysis for the first time the effects of optical phase changes on pupil apodisation which has the tendency to enhance velocity errors. We study the resolution and the velocity errors for the three-etalon filtergraph TESOS (Kentischer et al. \cite{kenti1}) at the Vacuum Tower Telescope on Tenerife. Peak-to-valley velocity errors for solar structure with sizes of 2 arcsec and less and a contrast of 15% amount to less than 50 m/s for the low resolution mode and to less than 10 m/s for the high resolution mode of TESOS in the worst case. We conclude that the performance of TESOS is not severely compromised by pupil apodisation. Title: 10-um interferometry on the VLTI with the MIDI instrument: a preview Authors: Leinert, Christopher; Graser, Uwe; Waters, Laurens B.; Perrin, Guy; Lopez, Bruno; Coudé du Foresto, Vincent; Glazenborg-Kluttig, Annelie W.; de Haas, Johannes C.; Herbst, Thomas M.; Jaffe, Walter; Lena, Pierre J.; Lenzen, Rainer; le Poole, Rudolf S.; Ligori, Sebastiano; Mundt, Reinhard; Pel, Jan-Willem; Porro, Irene L.; von der Luehe, Oskar Bibcode: 2000SPIE.4006...43L Altcode: We describe principles, design and present status of MIDI, the mid-infrared interferometric instrument for the VLTI, which is planned to come into operation at the ESO Very Large Telescope Interferometer during the second half of 2001. Title: Science opportunities with AMBER, the near-IR VLTI instrument Authors: Richichi, Andrea; Bloecker, Thomas; Foy, Renaud; Fraix-Burnet, Didier; Lopez, Bruno; Malbet, Fabien; Stee, Philippe; von der Luehe, Oskar; Weigelt, Gerd Bibcode: 2000SPIE.4006...80R Altcode: AMBER is the near-IR instrument for the VLTI, which will offer the possibility of combining two or three beams from either the 8 meter VLT main telescopes or the 1.8 meter auxiliary telescopes. With spectral dispersion up to 10,000 high visibility accuracy and the ability to obtain closure phases, AMBER will offer the means to perform high quality interferometric measurements in the 1 - 2.5 micron range initially, with later extensions to other portions of the spectrum. These design characteristics, coupled to the VLT interferometer potential, open up the access to investigation of several classes of objects, from stellar to extragalactic astronomy. We will review the projected performance in terms of sensitivity and angular resolution, and illustrate the potential applications in some key research areas. In particular, we will present the work of the AMBER Science Group, which is evaluating simulated data of source models and interferometric outputs for the purpose of defining the criteria for observations. Title: Kiepenheuer-Institut für Sonnenphysik. Jahresbericht für 1999. Authors: von der Lühe, O. Bibcode: 2000MitAG..83..209V Altcode: No abstract at ADS Title: GREGOR: A 1.5m Telescope for Solar Research Authors: von der Lühe, O.; Schmidt, Wolfgang; Soltau, Dirk; Kneer, Franz; Staude, Jurgen Bibcode: 2000ESASP.463..629V Altcode: 2000sctc.proc..629V No abstract at ADS Title: Solar Observation with High Spatial and Spectral Resolution Authors: Krieg, J.; Wunnenberg, M.; Kneer, F.; Koschinsky, M.; Ritter, C.; von der Lühe, O. Bibcode: 1999AGAb...15...88K Altcode: 1999AGM....15..P03K The solar atmosphere is, like stellar atmospheres in general, highly dynamic on small scales. To understand, e.g., the generation of turbulent motion and waves or chromospheric and coronal heating processes, observations with high resolution are needed. We achieve both high spatial and spectral resolution with a double Fabry-Perot interferometer which allows image restoration with speckle methods. Our new results include the height variation of the granular intensity pattern and velocity maps of unprecedented spatial resolution. Title: Estimated Performance for 10-micron Interferometry at the VLTI with the MIDI Instrument Authors: Porro, I. L.; Graser, U.; Leinert, C.; Lopez, B.; von der Lühe, O. Bibcode: 1999ASPC..194..325P Altcode: 1999wfoi.conf..325P The mid-infrared interferometric instrument (MIDI) is planned to become operative at the ESO Very Large Telescope Interferometer (VLTI) in the year 2001. As for any other interferometers, many effects contribute to reduce the fringe visibility of MIDI, and eventually to affect its scientific capabilities. We present a preliminary visibility budget for MIDI: this is currently used to provide requirements for the optical design and in the definition of the scientific performances of the instrument. The role of atmospheric and diffraction effects for interferometry at mid-infrared wavelengths is also discussed. Finally an overview of the scientific objectives of MIDI is presented. Title: Kiepenheuer-Institut für Sonnenphysik. Jahresbericht für 1998. Authors: von der Lühe, O. Bibcode: 1999MitAG..82..215V Altcode: No abstract at ADS Title: GREGOR, a 1.5 M Solar Telescope Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Kneer, F.; Staude, J. Bibcode: 1999AGAb...15....5V Altcode: 1999AGM....15..A01V We present the design of a new large solar telescope which is going to be installed at the Observatorio del Teide, in the tower that presently houses the 45cm Gregory-Coudé-Telescope. The new telescope has an aperture of 1.5 meters and its optical design is basically a Gregorian configuration. It will be an open telescope in an azimuthal mount. An adaptive optics system is incorporated in the optical design as well as a polarimetry package. The feasibility of lightweight optics for the primary mirror has been investigated in an industrial pre-study. The focal plane instrumentation will include a high resolution filter spectrometer similar to the existing TESOS instrument at the VTT and a new spectro-polarimeter for the visible and the near UV. The latter instrument is presently being developed jointly by the KIS and the High Altitude Observatory in Boulder, USA. Budget permitting, detailed (Phase-B) planning will start in 2000, and the telescope will be developed and built in 2002 and 2003 with first light in spring of 2004. Title: Adaptive Optics for the German Solar Telescopes Authors: Soltau, D.; Brunner, R.; von der Lühe, O. Bibcode: 1999AGAb...15....6S Altcode: 1999AGM....15..A03S Adaptive Optics is a precondition to get high resolution observations near the diffraction limit when the integration times become larger than a few milliseconds At the KIS there is a project to upgrade the Vacuum Tower Telescope at Tenerife with an adaptive optics system (KAOS = Kiepenheuer-Institut adaptives Optiksystem). The optical concept is discussed and first measurements with the KAOS wavefront sensor and their implications are presented. Considerations with respect to AO for the future GREGOR telescope are also discussed. Title: High-resolution observations with the German Vacuum Tower Telescope on Tenerife Authors: von der Lühe, O. Bibcode: 1998NewAR..42..493V Altcode: The Vacuum Tower Telescope (VTT) on Tenerife has been the main ground-based observing facility of the German solar community since the late 1980s. We review its main features, performance and potential for high-resolution solar physics. Title: Simultaneous observations of spicules with SOHO/CDS and the Fabry-Perot interferometer at the VTT Authors: de Pontieu, B.; von der Lühe, O.; Soltau, D.; Kentisher, Th. Bibcode: 1998ESASP.421...43D Altcode: 1998sjcp.conf...43D No abstract at ADS Title: The angular diameter of R Doradus: a nearby Mira-like star Authors: Bedding, T. R.; Zijlstra, A. A.; von der Luhe, O.; Robertson, J. G.; Marson, R. G.; Barton, J. R.; Carter, B. S. Bibcode: 1997MNRAS.286..957B Altcode: 1997astro.ph..1021B We find the angular diameter of R Doradus to be 57+/-5mas, exceeding that of Betelgeuse and implying that R Dor is larger in apparent size than every star except the Sun. R Dor is shown to be closely related to the Mira variables. We estimate an effective temperature of 2740+/-190K, a distance of 61+/-7pc, a luminosity of 6500+/-1400 Lsolar and a radius of 370+/-50 Rsolar. The characteristics of R Dor are consistent with it being near the edge of a Mira instability strip. We detect non-zero closure phases from R Dor, indicating an asymmetric brightness distribution. We also observed W Hydrae, a small-amplitude Mira, for which we find an angular diameter of 44+/-4mas. Title: Results of studies on the environmental factors affecting the VLTI performance Authors: Koehler, Bertrand; Leveque, Samuel A.; von der Luehe, Oskar Bibcode: 1997SPIE.2871..544K Altcode: In the last few years, we have undertaken a number of studies and experiments to assess the impact of various environmental factors on the performance of the ESO very large telescope interferometer (VLTI). The investigated topics include atmospheric turbulence, wind loads on the telescope structure, vibrations created by equipment, natural thermal variation, thermal load from electronics, natural and man-made seismic noise, as well as acoustic noise. A first report of this activity was given in a previous paper. This paper presents the final results obtained in 1995. The main outcome is the very good confidence that the VLT 8 m telescope and the infrastructure design is adequate for interferometric use at optical wavelengths down to the visible. Title: R Doradus: the biggest star in the sky. Authors: Bedding, T. R.; Robertson, J. G.; Marson, R. G.; Zijlstra, A. A.; von der Lühe, O. Bibcode: 1997Msngr..87...34B Altcode: No abstract at ADS Title: Optical path difference model for the Very Large Telescope Interferometer Authors: Leveque, Samuel A.; Koehler, Bertrand; von der Luehe, Oskar Bibcode: 1997SPIE.2871..556L Altcode: The optical path difference model (OPD model) determines where to position the delay lines in order to compensate for on-axis delay as seen from an astronomical object of known coordinates. This model is equivalent to a pointing model but applied to the interferometric delay. The objective is to reduce the time to search for fringes and to improve the delay lines blind tracking accuracy. This aspect is of prime importance not only when considering the overall operational efficiency of the interferometer but also its ability to quickly observe a set of program objects even after relocation of the auxiliary telescopes. The optical path difference model is based on a precise knowledge of the interferometer configuration by including a set of calibration measurements. This paper describes the main characteristics of the model and includes the results of a simulation developed to fit telescope axis misalignments which contribute to optical path difference errors. Title: Interferometry with the ESO Very Large Telescope Authors: von der Luehe, Oskar; Derie, Frederic; Koehler, Bertrand; Leveque, Samuel A.; Paresce, Francesco; Verola, Massimo Bibcode: 1997SPIE.2871..498V Altcode: The interferometric mode of the ESO very large telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8 m diameter and by several auxiliary telescopes of the 2 m class. While the position of the 8 m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 stations distributed on the top of the observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8 m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferometric mode is the high sensitivity due to the 8 m pupil of the main telescopes, with the potential for adaptive optics compensation in the near- infrared spectral regime. The VLT interferometer is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is initially funded, and more capability may be added later while experience is gained and further funding becomes available. The scientific program is now defined by a team which consists of a VLTI scientist at ESO and fifteen astronomers from the VLT community. ESO has recently decided to resume the construction of the VLTI which was delayed in December 1993, in order to achieve first interferometric fringes with two of the 8 m telescopes around the year 2000, and routine operation with 2 m auxiliary telescopes from 2003 onwards. This paper presents an overview of the recent evolution of the project and its future development. Title: A new plan for the VLTI. Authors: von der Lühe, O.; Bonaccini, D.; Derie, F.; Koehler, B.; Léve^Que, S.; Manil, E.; Michel, A.; Verola, M. Bibcode: 1997Msngr..87....8V Altcode: The VLTI should acquire first fringes around the turn of the century in order to ensure competitiveness with other interferometry programmes. Title: Simulations of Vlti/visa Imaging Observations of Young Stellar Objects at 2.2µm Authors: Ageorges, N.; von der Lühe, O. Bibcode: 1997svlt.work..349A Altcode: No abstract at ADS Title: An Introduction to Interferometry with the ESO Very Large Telescope Authors: von der Lühe, O. Bibcode: 1997svlt.work...13V Altcode: No abstract at ADS Title: Pupil Mask Interferometry of M8E-IR Authors: Löwe, M.; Stecklum, B.; von der Lühe, O.; Quirrenbach, A. Bibcode: 1997svlt.work..379L Altcode: No abstract at ADS Title: Interferometry with the ESO Very Large Telescope Authors: von der Lühe; Dérié, F.; Koehler, Bertrand; Lévêque, S.; Paresce, F.; Verola, M. Bibcode: 1997ioai.book....9V Altcode: No abstract at ADS Title: Stellar Surface Observations with the Vlti Authors: von der Lühe, O. Bibcode: 1997svlt.work..303V Altcode: No abstract at ADS Title: Results of Studies on the Environmental Factors Affecting the Vlti Performance Authors: Koehler, B.; Lévêque, S.; von der Lühe, O. Bibcode: 1997svlt.work..369K Altcode: No abstract at ADS Title: Internal Optical Path Calibration System for Vlti Authors: Lévêque, S.; Koehler, B.; von der Lühe, O. Bibcode: 1997svlt.work..375L Altcode: No abstract at ADS Title: Imaging in interferometry Authors: von der Lühe, O.; Ageorges, N. Bibcode: 1997ASIC..501...81V Altcode: 1997hara.conf...81V No abstract at ADS Title: Adaptive Optics at the German VTT on Tenerife Authors: Soltau, D.; Acton, D. S.; Kentischer, Th.; Roser, M.; Schmidt, W.; Stix, M.; von der Luhe, O. Bibcode: 1997ASPC..118..351S Altcode: 1997fasp.conf..351S Experiences and considerations are presented which play an important role for an Adaptive Optics system at our telescope. Title: Kilometric baseline space interferometry Authors: Bely, Pierre Y.; Laurance, Robin J.; Volonte, Sergio; Greenaway, Alan H.; Haniff, Christopher A.; Lattanzi, Mario; Mariotti, Jean-Marie; Noordam, Jan E.; Vakili, Farrokh; von der Luehe, Oskar; Lambert, Herve; Calvel, Bertrand; Scaddan, Richard J.; Vangasse, Paul Bibcode: 1996SPIE.2807...59B Altcode: Two versions of a kilometric interferometer with equivalent science capabilities have been studied, one located on the Moon and the other operating as a free-flyer. It has been found that the Moon is not the ideal site for interferometry because of tidal and micro-meteorite induced disturbances, the need for long delay lines and the large temperature swings from day to night. Automatic deployment of the Moon- based interferometer would be difficult and site preparation and assistance by man appear to be essential. The free-flyer would be implemented as a very accurately controlled cluster of independent satellites placed in a halo orbit around the 2nd Lagrange point of the Sun-Earth system. Both versions could attain the required scientific performances and each one needs the same type of metrology control. The free-flyer is intrinsically advantageous because of its reconfiguration flexibility, quasi-unlimited baseline length and observation efficiency (the Moon-based interferometer cannot be operated during the lunar day because of stray light). The free-flyer is better suited for implementation in the near or mid-term future, but the Moon-based version could be considered in the long term when a human presence would permit maintenance and upgrading leading to a longer lifetime with continuous performance enhancement. Title: Longitudinal Dispersion Compensation for the Very Large Telescope Interferometer Authors: Lévêque, Samuel; Koehler, Bertrand; von der Lühe, Oskar Bibcode: 1996Ap&SS.239..305L Altcode: The Very Large Telescope Interferometer [1,13] will be operated in air which will introduce small optical path fluctuations due to internal turbulence [4,6] and dispersion effects. Both may contribute to fringe contrast decrease. Longitudinal dispersion effects can be corrected by inserting an appropriate glass of variable thickness in one arm of the interferometer [3,5,11,12]. This paper presents a new method applied to VLTI in order to select the optimum glass, according to both the observing wavelength and the spectral bandpath, and to calculate its thickness. Results are presented in terms of improvement on the fringe contrast. Title: Simulations of VLTI/VISA imaging observations of young stellar objects at 2.2 μm. Authors: Ageorges, N.; von der Lühe, O. Bibcode: 1996Msngr..85...18A Altcode: No abstract at ADS Title: Kilometric baseline space interferometry. Comparison of free-flyer and moon-based versions. Report by the Space Interferometry Study Team. Authors: Bély, P. -Y.; Laurance, R. J.; Volonte, S.; Greenaway, A.; Haniff, C.; Lattanzi, M. G.; Mariotti, J. -M.; Noordam, J. E.; Vakili, F.; von der Lühe, O. Bibcode: 1996kbsi.book.....B Altcode: The authors have compared two versions of a kilometric interferometer with equivalent science capabilities, one located on the moon and the other operating as a free flying instrument. They have concluded that the free-flyer is better suited for an implementation in the near or mid-term future, but that the moon-based version should be considered in the long term in conjunction with a manned lunar infrastructure. Title: A new start for the VLTI. Authors: Paresce, F.; Mourard, D.; Bedding, T.; Beletic, J.; Haniff, C.; Leinert, C.; Malbet, F.; Mariotti, J. -M.; Mozurkewich, D.; Mundt, R.; Petitjean, P.; Quirrenbach, A.; Reinheimer, T.; Richichi, A.; Röttgering, H.; von der Lühe, O.; Waters, R. Bibcode: 1996Msngr..83...14P Altcode: The complexity and ambitious scope of VLTI mean that its astrophysical repercussions are difficult to define fully, even for many of its most vocal supporters. However, the primary scientic issues that it seeks to address are well defined, although there remains a need to present these coherently to the wider community in order to justify the significant resources which the project requires. Another pressing need is to develop an implementation plan that will optimally exploit the various technological stages of the project and ensure their compatibility with a vigorous, yet realistic and timely, astrophysical programme. Title: Astrometrie mit dem VLT interferometer der ESO. Authors: von der Lühe, O. Bibcode: 1996AGAb...12...76V Altcode: No abstract at ADS Title: Scientific and technical goals for solar adaptive optics. Authors: von der Lühe, O. Bibcode: 1996AGAb...12...86V Altcode: No abstract at ADS Title: Adaptive optics for a 70 cm solar telescope. Authors: Soltau, D.; Acton, D. S.; Kentischer, T.; Röser, M.; Schmidt, W.; Stix, M.; von der Lühe, O. Bibcode: 1996AGAb...12...92S Altcode: No abstract at ADS Title: Observing stellar surface structure with the ESO-VLT interferometer Authors: von der Luhe, O.; Solanki, S.; Reinheimer, Th. Bibcode: 1996IAUS..176..147V Altcode: No abstract at ADS Title: The Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory Authors: Kohl, J. L.; Esser, R.; Gardner, L. D.; Habbal, S.; Daigneau, P. S.; Dennis, E. F.; Nystrom, G. U.; Panasyuk, A.; Raymond, J. C.; Smith, P. L.; Strachan, L.; Van Ballegooijen, A. A.; Noci, G.; Fineschi, S.; Romoli, M.; Ciaravella, A.; Modigliani, A.; Huber, M. C. E.; Antonucci, E.; Benna, C.; Giordano, S.; Tondello, G.; Nicolosi, P.; Naletto, G.; Pernechele, C.; Spadaro, D.; Poletto, G.; Livi, S.; Von Der Lühe, O.; Geiss, J.; Timothy, J. G.; Gloeckler, G.; Allegra, A.; Basile, G.; Brusa, R.; Wood, B.; Siegmund, O. H. W.; Fowler, W.; Fisher, R.; Jhabvala, M. Bibcode: 1995SoPh..162..313K Altcode: The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The purpose of the UVCS instrument is to provide a body of data that can be used to address a broad range of scientific questions regarding the nature of the solar corona and the generation of the solar wind. The primary scientific goals are the following: to locate and characterize the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate the solar wind, to understand how the coronal plasma is heated in solar wind acceleration regions, and to increase the knowledge of coronal phenomena that control the physical properties of the solar wind as determined byin situ measurements. To progress toward these goals, the UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliocentric height of 12 solar radii. Title: Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory: instrument description and calibration overview Authors: Kohl, J. L.; Esser, R.; Gardner, Larry D.; Habbal, S.; Daigneau, P. S.; Nystrom, George U.; Raymond, John C.; Strachan, Leonard; van Ballegooijen, A. A.; Noci, G.; Fineschi, Silvano; Romoli, Marco; Ciaravella, A.; Modigliani, A.; Huber, Martin C.; Antonucci, E.; Benna, C.; Giordano, S.; von der Luehe, Oskar; Tondello, Giuseppe; Nicolosi, Piergiorgio; Naletto, Giampiero; Pernechele, Claudio; Geiss, J.; Gloeckler, G.; Poletto, G.; Spadaro, D.; Allegra, A.; Basile, G.; Brusa, R.; Wood, B.; Siegmund, Oswald H. Bibcode: 1995SPIE.2517...40K Altcode: The SOHO ultraviolet coronagraph spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliographic height of 12 R. In this paper, the salient features of the design of the UVCS instrument are described. An overview of the UVCS test and calibration activities is presented. The results from the calibration activity have demonstrated that the UVCS can achieve all its primary scientific observational goals. Title: Interferometry with the ESO Very Large Telescope Authors: von der Luehe, Oskar; Derie, Frederic; Ferrand, Didier; Koehler, Bertrand; Leveque, Samuel A.; Paresce, Francesco; D'Arcio, Luigi Bibcode: 1995SPIE.2566..124V Altcode: The interferometric mode of the ESO very large telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8m diameter and by several auxiliary telescopes of the 2m class. While the position of the 8m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 stations distributed on the top of the observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferrometric mode is the high sensitivity due to the 8m pupil of the main telescopes which will be compensated by adaptive optics in the near-infrared spectral regime. The VLT interferometer (VLTI) part of the VLT program is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is initially funded, and more capability may be added later while experience is gained and further funding becomes available. Major subsystems of the present baseline VLTI include: three auxiliary telescopes, three delay lines which permit combining the light from up to four telescopes, and a laboratory which contains an imaging beam combiner telescope, and enough space to accomodate a number of experimental setups. This paper presents a general overview of the recent evolution of the project and its future development. Title: Measurement of optical path fluctuations due to internal seeing for the VLTI Authors: Leveque, Samuel A.; Koehler, Bertrand; von der Luehe, Oskar Bibcode: 1995SPIE.2566..156L Altcode: The very large telescope interferometer (VLTI) will relay stellar beams from each individual telescope to the combining facility through an air path, as opposed to vacuum. Internal air turbulence will induce optical path fluctuations which have been taken into account in the global VLTI error budget and in the expected performance of the delay line control system. This paper presents experimental data used to validate the assessment of these turbulence effects. A comparison with theoretical models developed for the free atmosphere is included in order to investigate their applicability in the VLTI delay line tunnel. Optical path fluctuations were measured by a laser interferometer working in the 0-80m range, in an underground tunnel representative of the VLTI beam transport facilities. The derived index structure coefficient, Cn2, inside the tunnel has been compared with high sensitivity temperature measurements. Title: The Ultraviolet Coronagraph Spectrometer for the Solar and Hellospheric Observatory Authors: Raymond, J. C.; Kohl, J. L.; Esser, R.; Gardner, L. D.; Habbal, S.; Strachan, L.; van Ballegooijen, A. A.; Noci, G.; Fineschi, S.; Romoli, M.; Huber, M. C. E.; Antonucci, E.; Benna, C.; von der Luhe, O.; Naletto, G.; Nicolosi, P.; Pernechele, C.; Tondello, G.; Geiss, J.; Gloeckler, G.; Spadaro, D.; Daigneau, P. S.; Nystrom, G. U.; Allegra, A.; Basile, G.; Brusa, R.; Wood, B.; Siegmund, O. H. W. Bibcode: 1995SPD....26..720R Altcode: 1995BAAS...27..970R No abstract at ADS Title: Imaging the Surfaces of Late-Type Stars Authors: Bedding, T. R.; von der Lühe, O.; Zijlstra, A. A. Bibcode: 1995svlt.conf..100B Altcode: No abstract at ADS Title: Calibration of the VLT Interferometer Authors: von der Lühe, O.; Quirrenbach, A. Bibcode: 1995ESOC...53..173V Altcode: 1995cuhe.conf..173V No abstract at ADS Title: Narrow-Angle Astrometry with the VLT Interferometer Authors: von der Lühe, O.; Quirrenbach, A.; Koehler, B. Bibcode: 1995svlt.conf..445V Altcode: No abstract at ADS Title: Observation of Surface Activity on Cool Giants with the VLT Interferometer Authors: von der Lühe, O.; Schüssler, M.; Solanki, S. K.; Caligari, P. Bibcode: 1995svlt.conf...94V Altcode: No abstract at ADS Title: The MPE imaging beam combiner simulator COSI Authors: Boeker, T.; Cruzalebes, P.; Hofmann, R.; Katterloher, R.; Eckart, A.; Genzel, R.; Drapatz, S.; von der Luehe, O. Bibcode: 1994A&A...288..656B Altcode: We present first interferograms and reconstructed images obtained with the MPE imaging beam combiner simulator COSI. The purpose of COSI is to simulate the imaging beam combiner at the coherent focus of the ESO VLTI in multi-speckle mode or under conditions of partial or full correction of the single telescope wave front by adaptive optics. COSI consists of a 1 m telescope and a near-infrared continuum light source to simulate the radiation from astronomical objects. Two flat mirrors allow us to use one half of the telescope as a transmitter and the other half as a receiver. In the receiving focus we have installed the MPE speckle camera SHARP, which uses a HgCdTe 256^2^ NICMOS 3 array. A pupil mask over the aperture allows us to simulate various telescope configurations with a beam compression factor of 100 as it will be used for the ESO VLT interferometer. COSI is used to explore NIR array detector properties and their suitability for interferometric measurements and to generate data to explore image reconstruction algorithms. First interferograms of single and multiple objects were taken early this year (1993). Employing various deconvolution and Fourier-inversion methods, a diffraction limited image of the pin-hole sources can be successfully recovered which experimentally demonstrates the feasibility of interferometric imaging with a beam combiner. Thus, we have demonstrated that COSI is an excellent test bed to investigate methods of image recovery and to investigate how the methods are influenced by effects like atmospheric turbulence, expected optical imperfections and detector characteristics. Title: Interferometric mode of the European Southern Observatory Very Large Telescope Authors: von der Luehe, Oskar; Ferrand, Didier; Koehler, Bertrand; Zhu, N.; Reinheimer, Thorsten Bibcode: 1994SPIE.2200..168V Altcode: 1994aisi.conf..168V The interferometric mode of the ESO Very Large Telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8-m diameter and by several auxiliary telescopes of the 2-m class. While the position of the 8-m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 distributed on the top of the Observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8-m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferometric mode is the high sensitivity due to the 8-m pupil of the main telescopes which will be compensated by adaptive optics in the near-IR spectral regime. The VLT Interferometer (VLTI) part of the VLT Programme is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is funded, and more capability may be added later while experience is gained and further funding becomes available. Major subsystems of the present baseline VLTI include: three auxiliary telescopes, three delay lines which permit combining the light from up to four telescopes, and a laboratory which contains an imaging beam combiner telescope and enough space to accommodate a number of experimental setups. This paper presents a general overview of the recent evolution of the project and its future development. Title: Engineering aspects of the environmental factors affecting the VLTI performance Authors: Koehler, Bertrand; von der Luehe, Oskar Bibcode: 1994SPIE.2200..180K Altcode: 1994aisi.conf..180K The stringent and specific requirements associated with visible aperture synthesis projects call for a sound engineering effort in the design and development phase to assess the instrumental performance. An important area of effort concerns the influence of the natural or man-made environmental factors on the global performance of the interferometer. This paper discusses the major environmental factors affecting the Very Large Telescope Interferometer (VLTI) and presents the results of a number of studies aimed at evaluating the effects of such environmental factors. Title: Highly variable curvature mirrors for the Very Large Telescope Interferometer Authors: Ferrari, Marc; Lemaitre, Gerard R.; Mazzanti, Silvio; von der Luehe, Oskar; di Biagio, Bernard; Montiel, Pierre; Revest, Daniel; Joulie, Patrice; Carre, Jean-Francois Bibcode: 1994SPIE.2201..811F Altcode: The design of two holosteric configurations have been optimized for a maximum center/edge de-flexure of 400 micrometers . Their thickness distribution is given for active zones of 16 mm in diameter. The curvature action is obtained from an air-pressure chamber that generates onto the rear side of the mirrors (1) a uniform pressure up to 9 Atm or (2) a central force up to 11 daN. The control of the curvature is made by an accurate pressure gauge. Some preliminary results are shown as obtained on metal prototype VCMs from a first fabrication cycle as well as an X-ray device for testing the machining validity of the boundaries at the edge of mirrors. Title: Estimating residual aberrations from images taken at the user focus of a telescope compensated by adaptive optics Authors: von der Luehe, Oskar; Theodore, Bertrand Bibcode: 1994SPIE.2201..989V Altcode: We present a simple procedure which estimates aberrations in the exit pupil from point source images taken at the compensated focus. The procedure is based on an iterative technique described by J. Fienup. The measured point spread function of the system produced with a reference source at the input to the adaptive optics system, and the exact, properly scaled shape of the (centrally obscured) exit pupil is all that is needed for the procedure to perform. The results can be obtained quickly, e.g., as part of a daily maintenance procedure. We present some sample cases which were produced with the Come-On Plus adaptive optics system at the ESO 3.6 m telescope on La Silla. Title: Speckle imaging of solar small scale structure. 2: Study of small scale structure in active regions Authors: von der Luehe, O. Bibcode: 1994A&A...281..889V Altcode: The speckle imaging technique which is described in the first paper of this series (von der Luehe 1993) was used to analyze time series of high angular resolution images of solar small scale structure at a wavelength of 585 nm in active regions with the 76 cm diameter vacuum tower telescope at National Solar Observatory (NSO)/Sac Peak. Two sets of reconstructed images with a field of 4 by 4 arcsec which cover a period of 36 min and 83 min were generated and analyzed. The image reconstructions are supplemented with simultaneous large field photographs taken within a 15 A passband centered on the Ca II K (3933) line. The prime objective of the observing program was the study of the structure and the dynamics of the continuum wavelength counterpart of facular points which appear with high contrast in the Ca pictures, i.e., continuum bright points (CBPs). In addition to CBPs, the reconstructions allow studying other small scale phenomena. Results of the studies are given. Title: First Images with the MPE Imaging Beam Combiner Simulator COSI Authors: Böker, T.; Cruzalèbes, P.; Hofmann, R.; Katterloher, R.; Eckart, A.; Genzel, R.; Drapatz, S.; Beckers, J.; von der Lühe, O.; Merkle, F. Bibcode: 1994ESOC...48..161B Altcode: 1994aao..conf..161B No abstract at ADS Title: The VLT Interferometer [invited] Authors: Bedding, T. R.; Beckers, J. M.; Faucherre, M.; Hubin, N.; Koehler, B.; von der Lühe, O.; Merkle, F.; Zhu, N. Bibcode: 1994IAUS..158..143B Altcode: No abstract at ADS Title: Narrow-Band Speckle Imaging Authors: Keller, C.; von der Lühe, O. Bibcode: 1993rtpf.conf..129K Altcode: No abstract at ADS Title: First light from the NTT interferometer. Authors: Bedding, T. R.; von der Lühe, O.; Zijlstra, A. A.; Eckart, A.; Tacconi-Garman, L. E. Bibcode: 1993Msngr..74....2B Altcode: No abstract at ADS Title: Imaging of the Solar Surface with Interferometric Arrays Authors: von der Lühe, O. Bibcode: 1993rtpf.conf..150V Altcode: No abstract at ADS Title: Speckle imaging of solar small scale structure. I - Methods Authors: von der Luehe, O. Bibcode: 1993A&A...268..374V Altcode: Speckle interferometry and imaging methods can improve considerably the angular resolution of images observed with a solar telescope. This paper describes in detail an implementation of the Knox-Thompson speckle imaging technique which has been developed specifically for the needs of solar imaging, in order to produce photometrically accurate maps of the intensity of solar small-scale features. The technique can recover fields of view of arbitrary size, i.e., fields which may be much larger than the isoplanatic patch. A number of problems which are peculiar to solar observations have been identified and were solved in the course of development of the imaging procedures. These problems and their solution are also described in detail. It is demonstrated that the application of speckle imaging to solar data results in near-diffraction-limited performance with a 76 cm telescope at a wavelength of 600 nm +/- 3 nm when seeing conditions are good, but not necessarily exceptionally good. Title: Solar speckle polarimetry Authors: Keller, C. U.; von der Luehe, O. Bibcode: 1992A&A...261..321K Altcode: The combination of a polarimeter with real-time frame selection and differential speckle imaging results in diffraction-limited magnetograms that provide new insight into the morphology of solar small-scale magnetic fields. The method to record diffraction limited narrow-band filtergrams of solar features is based on two cameras taking simultaneous short exposure images through a broad-band and a narrow-band filter, respectively. Speckle imaging reconstructs the image in the broad-band channel. This reconstruction determines the instantaneous optical transfer function (OTF) for each individual broad-band exposure. Each simultaneously recorded image in the narrow-band channel is then corrected for the instantaneous OTE To recover all spatial frequencies in the narrow-band channel the so corrected single images are averaged. We have applied the method to polarimetric observations of a solar active region by tuning the narrow-band filter to the wing of a Zeeman sensitive spectral line. The most active part of the region shows no more normal granules but features with a diameter of 0.3 to 0.5 arcsec. The smallest magnetic fields are concentrated in regions with sizes at or below the diffraction limit of the telescope. Title: Application of Differential Speckle Imaging to Solar Polarimetry Authors: Keller, C. U.; von der Luhe, O. Bibcode: 1992ESOC...39..453K Altcode: 1992hrii.conf..453K No abstract at ADS Title: The Configuration of the VLT Interferometer on the Paranal Site Authors: von der Luhe, O.; Beckers, J. M.; Braun, R. Bibcode: 1992ESOC...39..959V Altcode: 1992hrii.conf..959V No abstract at ADS Title: A Progress Report on the Implementation of the VLT Interferometer Authors: Beckers, J. M.; Faucherre, M.; Koehler, B.; von der Luhe, O. Bibcode: 1992ESOC...39..775B Altcode: 1992hrii.conf..775B No abstract at ADS Title: Coudé Near Infrared Camera Instrument Contract Signed Authors: Lenzen, R.; von der Lühe, O. Bibcode: 1992Msngr..67...17L Altcode: No abstract at ADS Title: High Spatial Resolution Magnetograms of Solar Active Regions Authors: Keller, C. U.; Stenflo, J. O.; von der Luhe, O. Bibcode: 1992A&A...254..355K Altcode: Using the Universal Birefringent Filter at the Sacramento Peak Vacuum Tower Telescope we have obtained simultaneous observations of left and right circular polarization in various solar magnetic features with a resulting spatial resolution of 0".7 in the magnetograms. We describe the data reduction in some detail and discuss the various instrumental effects. In particular we show that seeing can create features in magnetograms. A penumbra near disk center shows small-scale features in the magnetogram which are associated with the bright filaments. Bright features in the umbra of a small spot exhibit considerable polarization signals. In a pore region opposite polarities are found within a few seconds of arc. Title: High spatial resolution techniques. Authors: von der Luehe, O. Bibcode: 1992soti.book....1V Altcode: Contents: (1) Introduction. (2) Basic considerations and concepts: diffraction and optical transfer; coherence, Van Cittert-Zernike theorem. (3) Wave propagation through the atmosphere: statistics of index of refraction fluctuations; mutual intensity of a wave disturbed by turbulence; structure functions; the instantaneous optical transfer function. (4) Single frame analysis: data collection and preparation; image selection; single picture restoration; time series analysis. (5) Interferometry: Michelson interferometry; interferometric arrays; speckle interferometry; the Labeyrie method; seeing calibration; noise calibration; speckle imaging, Knox-Thompson; speckle imaging, speckle masking; speckle interferometry and anisoplanatism. (6) Active wavefront compensation: image motion compensation; adaptive optics. (7) Other methods: radio observations; high resolution observations from space; solar optical universal polarimeter; orbiting solar laboratory; solar and heliospheric observatory. Title: Solar feature correlation tracker. Authors: Rimmele, Th.; von der Luehe, O.; Wiborg, P. H.; Widener, A. L.; Dunn, R. B.; Spence, G. Bibcode: 1991SPIE.1542..186R Altcode: The authors present a tracking system that stabilizes atmospheric and instrumental image motion at the vacuum tower telescopes of the National Solar Observatory at Sacramento Peak and the Kiepenheuer Institut für Sonnenphysik at Tenerife. A matrix diode array rapidly scans the scene of interest, usually with a field of 5 arcsec. Images are cross-correlated in real time with a previously recorded reference image of the same area. Reference pictures are updated every 30 s. Recent performance tests show that the residual image motion in the tracked image is 0.05 arcsec rms compared to a typical 0.5 arcsec rms for the untracked image. The correlation tracker also includes a seeing monitor providing a relative seeing measure at a two millisecond rate, which can be used for frame selection and shutter control. Title: LEST Mini-Workshop: Entrance windows of solar telescopes. Summary and conclusions. Authors: von der Lühe, O. Bibcode: 1991ewst.conf...47V Altcode: The main conclusion that can be drawn from the workshop is that the LEST entrance window is no longer a critical factor of the telescope design. There is a lot of confidence that the window is feasible, and as a result the discussions have concentrated on special issues and have gone into considerable detail. The presentations given at the workshop can be categorized into four broad topics: manufacture of the window, control of the window surface reflectivity, thermal control of the window, and polarization effects. The author will address each of the topics in this sequence. Title: Coronal observations with SOHO Authors: Huber, Martin C. E.; von der Luehe, Oskar Bibcode: 1991AdSpR..11a.339H Altcode: 1991AdSpR..11..339H The Solar and Heliospheric Observatory (SOHO) will carry a set of solar physics experiments which permit a thorough investigation of the solar corona. The emphasis of the mission is on the measurement of the physical properties of coronal structures and the processes occurring therein, leading - it is hoped - to an understanding of the mechanism(s) by which the solar corona is heated and the solar wind is accelerated. The observations will be made from a halo orbit around the L1 Lagrange point on the Earth-Sun line and will range from magnetic field measurements in the photosphere through spectroscopic plasma diagnostics of chromospheric, transition-zone and coronal structures with high spatial and spectral resolution, to coronagraphic observations out to 30 solar radii and to mass spectrometry of the solar wind near one astronomical unit.

SOHO is part of the Solar Terrestrial Science Programme (STSP), the first `Cornerstone' in ESA's long-term scientific programme `Space Science - Horizon 2000', and will therefore be flown in connection with the other STSP component, the four-spacecraft Cluster mission, which will investigate plasma structures and processes in the magnetosphere in three dimensions.

Both STSP missions, i.e. SOHO and Cluster will address the physics of plasma structures and processes, that are accessible to investigation in the solar-terrestrial context, yet are thought to be examples of plasma processes and structures that are ubiquitous in the cosmos. It is hoped that a cross-fertilisation between the scientific communities associated with SOHO and Cluster will take place, as they investigate the physics of the coronal and magnetospheric plasma with complementary methods and techniques - globally by remote observations, and in detail, by multi-point in-situ measurements. Title: High spatial resolution techniques. Authors: von der Luehe, O. Bibcode: 1991sia..book..688V Altcode: Observations of the solar surface from the ground are severely affected by turbulence of the Earth's atmosphere. Beside selecting observatory sites for excellent atmospheric quality, the careful design of the observing equipment, and the application of modern methods that undo the effects of the atmosphere or recover degraded data provide a wealth of information on small-scale processes. The effects of atmospheric turbulence on solar observations are reviewed, and methods that achieve high angular resolution on the Sun, such as adaptive optics and interferometry, are discussed. Some experiments that provide solar observations of high spatial resolution are presented. Title: Solar adaptive optics Authors: von der Luehe, O. Bibcode: 1991AdSpR..11e.275V Altcode: 1991AdSpR..11..275V Ground-based observations of solar small-scale structure is severely degraded by thermal fluctuations in the Earth's atmosphere. Adaptive optics systems can compensate in real time the aberrations caused by seeing, permitting observations close to the diffraction limit of the telescope. There are adaptive optical systems with varying degrees of complexity, ranging from simple fast guiders for high-contrast, confined targets to sophisticated systems that undo high degree wavefront deformations. This paper reviews the main principles of adaptive optics, addresses issues peculiar to solar adaptive systems, and presents current developments in this area. Title: Adaptive Optik für die Sonnenbeobachtung. Authors: Rimmele, Thomas; von der Luehe, Oskar Bibcode: 1990S&W....29..520R Altcode: The present status is reviewed of efforts to construct an earth-based solar observatory which will compensate for the distorting effects of the earth's atmosphere on solar observations. Image movement compensation techniques are described, including the recently developed Correlation Tracker. Efforts being made in the area of higher-order adaptive optics are addressed. Title: Solar feature correlation tracker for ground-based telescopes Authors: von der Luehe, O.; Widener, A. L.; Rimmele, Th.; Spence, G.; Dunn, R. B. Bibcode: 1989A&A...224..351V Altcode: A tracking system that stabilizes atmospheric and instrumental image motion has been tested at the vacuum tower telescope of the National Solar Observatory at Sacramento Peak. The system locks anywhere on the sun, using solar granulation or other small scale structures as tracers. A matrix diode array rapidly scans the scene of interest; pictures are cross-correlated in real time with a previously recorded reference image of the same area on the sun. Title: Solar Ultraviolet Network: an interferometric investigation of the fundamental solar astrophysical scales Authors: Dame, Luc; Moreau, Bernard G.; Cornwell, Timothy J.; Visser, H.; Title, Alan M.; Acton, Loren W.; Aime, Claude; Braam, Bart M.; Bruner, Marilyn E.; Connes, Pierre; Faucherre, Michel; Foing, B. H.; Haisch, Bernhard M.; Hoekstra, Roel; Heyvaerts, Jean; Jalin, Rene; Lemaire, Philippe; Martic, Milena; Muller, R.; Noens, J. C.; Porteneuve, Jacques; Schulz-Luepertz, E.; von der Luehe, Oskar Bibcode: 1989SPIE.1130..126D Altcode: The Solar UV Network (SUN) presently proposed is an interferometric system, based on the principles of stabilized interferometry, which will be capable of solar observations with spatial resolutions better than 0.013 arcsec. SUN will consist of four 20-cm diameter telescopes aligned nonredundantly on a 2-m baseline. SUN is judged to be ideally deployable by the NASA Space Station, if implemented on a pointing platform whose performance is of the order of the Instrument Pointing System flown on Spacelab 2. The compact, nonredundant configuration of SUN's telescopes will allow high-resolution imaging of a 2 x 2 arcsec field on the solar disk. Title: Prospects for solar interferometry. Authors: von der Lühe, O.; Zirker, J. B. Bibcode: 1989hsrs.conf..191V Altcode: No abstract at ADS Title: Solar speckle imaging. Authors: von der Lühe, O. Bibcode: 1989hsrs.conf..147V Altcode: This paper discusses the application of extensions of Labeyrie's speckle interferometry method to solar observations. These extensions fully reconstruct a picture of the source. Title: High spatial resolution solar observations Authors: von der Luehe, Oskar Bibcode: 1989hsrs.conf.....V Altcode: No abstract at ADS Title: Technical aspects of the speckle masking phase reconstruction algorithm Authors: Pehlemann, E.; von der Luehe, O. Bibcode: 1989A&A...216..337P Altcode: Technical problems that arise when the full four-dimensional speckle masking algorithm is implemented to reconstruct diffraction-limited two-dimensional images of astronomical objects are discussed, with particular emphasis on extended sources. The symmetries of the four-dimensional speckle masking bispectrum are used to relax computer memory requirements of the algorithm without loss of information. Further limits on the bispectrum reduce it to manageable sizes, but imply a loss of information. Various approaches of bispectrum truncation and the consequences for the phase recovery process are discussed, phase consistency and phase number diagrams are introduced as tools for quality assessment of the algorithm and the reconstruction. Different sequences for the phase recovery process are discussed as well. Sample reconstructions of a point source and of an extended object (a section of the solar photosphere) are presented. Title: A Correlation Tracker for Solar Fine Scale Studies. Authors: Rimmele, Th.; von der Luehe, O. Bibcode: 1989RvMA....2..105R Altcode: No abstract at ADS Title: High-resolution imaging of the solar photosphere. Authors: von der Lühe, O.; Pehlemann, E. Bibcode: 1989AGAb....2...34V Altcode: No abstract at ADS Title: A correlation tracker for solar fine scale studies. Authors: Rimmele, T.; von der Lühe, O. Bibcode: 1989amts.conf..105R Altcode: A solar feature correlation tracker was designed, built, and successfully tested in a joint effort of the National Solar Observatory in Sunspot, USA, and the Kiepenheuer Institut, Freiburg, Germany. The purpose of the system is stabilizing image motion which is caused by telescope shake and by seeing at the post-focus instruments of vacuum tower telescopes in Sunspot and in Izana. The tracker system features a matrix diode array as detector, fast digital processors, and an agile mirror as the optical active element. The processor consists of commercial and in-house built hardware. Title: Wavefront error measurement technique using extended, incoherent light sources Authors: von der Luhe, O. Bibcode: 1988OptEn..27.1078V Altcode: A technique for measuring wavefront errors in an optical system that receives light from a spatially extended, arbitrarily structured, incoherent source is described. If a suitable transparent mask is placed at an image plane of the system, the structure in the light source serves as a tracer for wavefront errors. The slope of the wavefront error can be detected in the form of intensity variations in a pupil image that follows the mask. One-dimensional numerical simulations of the method as well as the analytical treatment of the proposed principle are presented. The application of the technique as a wavefront sensor in an adaptive optical system for solar observations, in which aberrations are caused by atmospheric turbulence in the light path, is discussed as an example. Title: Measurements of characteristics of image motion with a solar image stabilizing device Authors: von der Luhe, O. Bibcode: 1988A&A...205..354V Altcode: An image motion stabilizer that locks on confined areas which are brighter or darker than the surrounding photosphere was used to measure some characteristics of atmospheric image motion at the Vacuum Tower telescope of the National Solar Observatory. The device stabilizes images of sunspots, pores, and faculae. Recordings of the drive signals were analyzed. Image motion between 0.3 and 0.5 arcsec rms was measured, power spectra of the recordings show roughly the behavior predicted theoretically for frequencies below 1 Hz and a much steeper decrease for frequencies larger than 5 Hz. Residual motion measured at various distances away from the stabilized area shows an increase over some 100 arcsec. The residual rms motion is less than the theoretical resolution of the telescope within roughly 30 arcsec around the stabilized area. Title: Signal transfer function of the Knox-Thompson speckle imaging technique Authors: von der Luhe, Oskar Bibcode: 1988JOSAA...5..721V Altcode: 1988OSAJ....5..721V The transfer function associated with the Knox-Thompson speckle imaging technique is investigated. Numerical model transfer functions using log-normal statistics for perturbation of the complex wave front, the near-field approximation, and a Kolmogorov spectrum for atmospheric turbulence statistics are presented. Simple approximations for the transfer function are discussed. As with the transfer function of Labeyrie's speckle interferometry technique, the portion beyond the seeing limit can be represented as the transfer function of an unaberrated telescope times a seeing-dependent constant. An additional factor depends on the frequency shift of the Knox-Thompson cross spectra. The influence of the frequency shift on the reconstructed phase error is discussed for simple reconstruction problems. Title: Scientific goals for solar interferometry. Authors: von der Lühe, O.; Zirker, J. B. Bibcode: 1988ESOC...29...77V Altcode: 1988hrii.conf...77V The authors review scientific programs for solar research that can be carried out using interferometric techniques. They establish boundary conditions for interferometric experiments. The performance and achievable resolution of interferometry is studied using an empirical model spectrum of the intensity fluctuation of solar granualtion as an example of extended, low-contrast small scale structure. The authors conclude that, with interferometric techniques, a ten-fold increase in resolution beyound today's seeing-limited performance is possible. Title: Speckle masking imaging of extended sources. Authors: von der Lühe, O.; Pehlemann, E. Bibcode: 1988ESOC...29..159V Altcode: 1988hrii.conf..159B The authors have developed a computer code to calculate speckle masking image reconstructions from two-dimensional observations of extended sources. The code was applied to specklegrams of solar small scale structure. The authors report on first results obtained with this code and compare reconstructions with those obtained using the Knox-Thompson technique. Title: First results with the NOAO 2-D speckle camera for infrared wavelengths. Authors: Beckers, J. M.; Christou, J. C.; Probst, R. G.; Ridgway, S. T.; von der Lühe, O. Bibcode: 1988ESOC...29..393B Altcode: 1988hrii.conf..393B The authors have constructed an infrared speckle camera using the NOAO SBRC 58×62 InSb array detector. The camera and detector can be used anywhere in the 1 to 5 μm wavelength region. The authors describe the first observations. These observations illustrate the performance of the camera for an unresolved object, for a double star (ζ Aqr) and for a resolved object (the "Red Rectangle"). The authors have analyzed the observations using shift-and-add, power spectrum/autocorrelation function, and Knox-Thompson methods. Title: Solar granulation power spectra from speckle interferometry Authors: von der Luhe, O.; Dunn, R. B. Bibcode: 1987A&A...177..265V Altcode: Granulation was observed with a CCD array at the SPO tower telescope in a quiet region near the solar center. Time series of short exposure (4 ms) pictures of a 14 by 14 arcsec region were taken. Consecutive frames were separated by 0.55 s in time. Labeyrie's (1970) speckle interferometry technique was applied in order to recover power spectra of the intensity fluctuations at high spatial frequencies. The spectral ratio technique (von der Luhe, 1984) was applied to correct for seeing attenaution of the power spectra. It is possible to recover a signal up to spatial frequency of 2.5 line pairs per arcsec, corresponding to a wave number of 22/Mm or 40 percent of the diffraction limit of the telescope under 1.3 arcsec average seeing conditions. A corrected rms granulation contrast of 0.127 + or - 0.01 is estimated, and an exponential falloff of power density toward higher spatial frequencies is observed. Title: Photon noise analysis for a LEST multidither adaptive optical system. Authors: von der Lühe, O. Bibcode: 1987LFTR...28..255V Altcode: A simple analysis to assess the influence of photon noise in a solar adaptive optic based on a multidither wavefront detection principle is carried out. It is argued that the performance should be practically independent of the type of control (modal or zonal). The performance of such a system increases slightly with the telescope diameter. It should be possible to control a few dozen modes or zones with a system working at a 2.4m diameter LEST. Title: Application of the Knox-Thompson Method to Solar Observations Authors: von der Luhe, O. Bibcode: 1987iia..conf...37V Altcode: No abstract at ADS Title: Photospheric Fine Structure Close to a Sunspot Authors: von der Lühe, O. Bibcode: 1987rfsm.conf..156V Altcode: Facular points, which typically have angular scales smaller than one arc second, are believed to represent the footpoints of magnetic flux tubes in the photosphere. The author made observations of facular points in the continuum and tried to resolve them using the Knox-Thompson speckle imaging technique. The objective was to measure the spatial extent of facular points and, if possible, to resolve their internal structure. Also, by using a time series of images, the lifetime of facular points can be determined and their interaction with the surrounding medium can be examined. Title: A wavefront sensor for extended, incoherent targets. Authors: von der Lühe, O. Bibcode: 1987LFTR...28..155V Altcode: The author proposes a novel method for sensing wavefront errors in an aberrated solar telescope. The wavefront sensor consists of a transparent mask located at an image plane. A difference image of the scene under observation is encoded in density on the mask. Wavefront error slopes can be detected in a pupil image following the mask in the form of intensity variations. The principle of the method is described and results of one-dimensional simulation calculations are presented. It is demonstrated that the proposed method is very sensitive and requires only a few percent of the incident light for a photon-noise limited null state measurement. Title: Study of Sizes Brightnesses and Dynamics of Solar Facular Points Authors: von der Luhe, O. Bibcode: 1987iia..conf..225V Altcode: This paper presents first results of an ongoing project to study the structure and the dynamics of small faculae. Title: Calibration Problems in Solar Speckle Interferometry Authors: von der Luhe, O. Bibcode: 1987iia..conf....9V Altcode: No abstract at ADS Title: Solar Image Stabilization Authors: von der Luhe, O. Bibcode: 1986BAAS...18..924V Altcode: No abstract at ADS Title: Initial Solar Observations at Sacramento Peak Using the Lockheed Active Optics System Authors: Smithson, R. C.; Sharbaugh, R. J.; Ramsey, H. E.; Acton, D. S.; Pari, M.; Keil, S. L.; Radick, R. R.; Simon, G. W.; von der Luehe, O.; Zirker, J. B. Bibcode: 1986BAAS...18..933S Altcode: No abstract at ADS Title: Speckle Image Reconstructions of Solar Pore Images taken in Hα 6563 Authors: Conde, A. L.; von der Luhe, O.; Radick, R. R. Bibcode: 1986BAAS...18R.933C Altcode: No abstract at ADS Title: Image Enhancement Techniques Applied to Solar Granulation Data Authors: von der Luehe, O.; Dunn, R. B.; November, L. J. Bibcode: 1986BAAS...18..663V Altcode: No abstract at ADS Title: The Speckle Masking Transfer Function Authors: von der Luhe, O. Bibcode: 1985A&A...150..229V Altcode: The transfer function for the speckle masking image reconstruction technique (Weigelt, 1977; Weigelt et al., 1983; Lohman et al., 1983 has been analyzed. The regions relevent to the speckle masking signal turn out to be proportional to (r0/D)4, where r0 is Fried's seeing parameter and D is the diameter of the telescope entrance pupil. Title: Adaptive Image Stabilization of Solar Observations: a Review Authors: von der Luehe, O. Bibcode: 1985LNP...233...62V Altcode: 1985hrsp.proc...62V Active wavefront correction techniques for solar imaging are discussed. The basic elements of all systems include: a wavefront error detector, servo logic, and an active optical element. The use of image motion (IM) control in image displacement detection and active element design for adaptive control systems is analyzed. The spot and correlation tracking methods of displacement detection are studied. The spot tracking uses a four-point limit guider, is based on local intensity maxima, and is applicable for observing sunspots and pores. The correlation trackers are designed to continuously scan an image detector and to compare the image with reference images in order to obtain the error signals. The application of agile, high quality mirrors either piezoelectrically or electrodynamically driven, to IM control is investigated. Research is being conducted on utilizing a real-time atmospheric compensation system in wavefront correction. Hardy's (1980) model, which consists of a rotational shearing interferometer as the wavefront sensor, monolithic piezoelectric mirrors as the active element, and analog logic to generate the drive signal, and the Smithson (1983) design, which uses a wavefront sensor that is a combination of the Hartmann test principle and the spot tracking principle, are examined. Title: High Resolution Speckle Imaging of Solar Small-Scale Structure: the Influence of Anisoplanatism Authors: von der Luehe, O. Bibcode: 1985LNP...233...96V Altcode: 1985hrsp.proc...96V The effects of anisoplanatism on image reconstructions developed with the Knox and Thompson (1974) technique are investigated. The procedures for reconstruction are described. A time series of 30 solar granulation images covering 14 x 14 arcsec and 15 s of time were analyzed. The four reconstructions derived from the series are: (1) the entire field centered in an area of 32 x 32 pixels, (2) the series without anisoplantic image motion, (3) the field restricted to the center 64 x 64 pixels covering approximately 7 x 7 arcsec, and (4) the center 40 x 40 pixels corresponding to a 4 x 4 arcsec square. Consistency tests were performed on the reconstructions. The comparisons of the reconstructions with a sample frame reveal that the Knox and Thompson technique is not useful when the field is larger than the isoplantic patch. Title: Estimating Fried's parameter from a time series of an arbitrary resolved object imaged through atmospheric turbulence. Authors: von der Luehe, O. Bibcode: 1984JOSAA...1..510V Altcode: 1984OSAJ....1..510V A method to obtain an estimate of Fried's seeing parameter r0 from time series of an arbitrarily shaped, resolved structure that exhibits degradation resulting from atmospheric turbulence is presented. The basic idea is to evaluate the ratio of the observed squared modulus of the average Fourier transform and the observed average power spectrum. The theory of the method is developed, and the influence of noise on the ratio is discussed. The method has been applied to five consecutive time series of observations of solar granulation under different seeing conditions. The power spectra, which are reconstructed with appropriate theoretical modulation transfer functions, converge. Title: A Method to Estimate Fried's Seeing Parameter from a Time Series of Arbitrary Resolved Structures Imaged Through the Atmosphere Authors: von der Luhe, O. Bibcode: 1984vlti.conf..203V Altcode: 1984IAUCo..79..203V A method is presented that allows to estimate the effective MTF from the observation of arbitrary structure with the use of Fried-Korff theory. The ratio of the squared modulus of the average Fourier transform and the average power spectrum serves as an estimator for the Fried parameter r0. To a first approximation, this ratio is independent from the observed object. Additionally, the behaviour of the ratio in regions beyond the seeing limit in the Fourier plane may be analyzed to obtain an estimate of the speckle interferometry signal-to-noise ratio. Title: A study of a correlation tracking method to improve imaging quality of ground-based solar telescopes Authors: von der Luehe, O. Bibcode: 1983A&A...119...85V Altcode: The measurement and correction of image motion introduced by the earth's atmosphere is undertaken, in order to improve solar imaging, by means of a correlation-tracking method designed to guide ground-based solar telescopes during the study of small scale, low contrast photospheric structures. A time series of digitized images was obtained with a two-dimensional CCD camera of 32 x 32 pixels. Image motion was determined from the locations of the cross-correlation function peaks of an arbitrarily chosen reference image, and the tracking method was simulated off-line by computer methods. Analysis results indicate that image motion may be suppressed to a residual rms value as low as 1/18th of the theoretical Rayleigh limit of telescopic resolution. The correlation method is found to work on arbitrary structures with rms constant values as low as a few percent under acceptable seeing conditions. Title: Adaptive optical systems for LEST. Authors: von der Lühe, O. Bibcode: 1983LFTR....2.....V Altcode: Some fundamental properties of adaptive optical systems and some basic design considerations for such a system on LEST have been discussed. Title: A comparison of optical and digital Fourier transformation of solar granulation Authors: von der Luehe, O. Bibcode: 1981A&A...101..277V Altcode: The application of an optical Fourier transformation setup to solar granulation transparencies is examined. An analysis of the photographic process makes it possible to derive calibrated radial power spectra of the granulation intensity fluctuations. The results are compared with power spectra of the same pictures obtained via microdensitometry and fast Fourier algorithms. It is found that the two independent methods lead to essentially the same quantitative results in the medium to high spatial wavenumber regions (2.5 to 10 per Mm). Disturbances due to the film supporting the granulation transmission pattern do not permit any significant optical power density estimates in the lower wavenumber regions (below 2.5 per Mm). It is concluded, however, that optical Fourier transformation, may be a useful alternative to other techniques in this field. Title: Ein Vergleich zwischen optischer und digitaler Fouriertransformation von Sonnengranulationsaufnahmen Authors: von der Lühe, O. Bibcode: 1981MitAG..54..261V Altcode: No abstract at ADS