Author name code: soltau ADS astronomy entries on 2022-09-14 author:"Soltau, Dirk" ------------------------------------------------------------------------ Title: Adaptive optics upgrades for laser communications to the ESA optical ground station Authors: Fischer, E.; Kudielka, K.; Berkefeld, T.; Soltau, D.; Perdigués-Armengol, J.; Sodnik, Z. Bibcode: 2021SPIE11852E..2AF Altcode: Over the past years, Synopta have successfully applied adaptive optics (AO) in optical ground stations to enhance the capabilities in satellite-to-ground laser communication links. In this paper, we present the outcome of an upgrade activity to the ESA Optical Ground Station (ESA OGS). The activities comprise the design, fabrication and site acceptance test results of the Cassegrain Adaptive Receiver Optics ("CARO") add-on instrument to the 1 m telescope of the ESA OGS. Additionally, we present a baseline design concept for the future use of a bi-directional adaptive optics system in the Coudé path of the ESA OGS, expected performance characteristics for a selected uplink application. Title: The Polarimetric and Helioseismic Imager on Solar Orbiter Authors: Solanki, S. K.; del Toro Iniesta, J. C.; Woch, J.; Gandorfer, A.; Hirzberger, J.; Alvarez-Herrero, A.; Appourchaux, T.; Martínez Pillet, V.; Pérez-Grande, I.; Sanchis Kilders, E.; Schmidt, W.; Gómez Cama, J. M.; Michalik, H.; Deutsch, W.; Fernandez-Rico, G.; Grauf, B.; Gizon, L.; Heerlein, K.; Kolleck, M.; Lagg, A.; Meller, R.; Müller, R.; Schühle, U.; Staub, J.; Albert, K.; Alvarez Copano, M.; Beckmann, U.; Bischoff, J.; Busse, D.; Enge, R.; Frahm, S.; Germerott, D.; Guerrero, L.; Löptien, B.; Meierdierks, T.; Oberdorfer, D.; Papagiannaki, I.; Ramanath, S.; Schou, J.; Werner, S.; Yang, D.; Zerr, A.; Bergmann, M.; Bochmann, J.; Heinrichs, J.; Meyer, S.; Monecke, M.; Müller, M. -F.; Sperling, M.; Álvarez García, D.; Aparicio, B.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Cobos Carracosa, J. P.; Girela, F.; Hernández Expósito, D.; Herranz, M.; Labrousse, P.; López Jiménez, A.; Orozco Suárez, D.; Ramos, J. L.; Barandiarán, J.; Bastide, L.; Campuzano, C.; Cebollero, M.; Dávila, B.; Fernández-Medina, A.; García Parejo, P.; Garranzo-García, D.; Laguna, H.; Martín, J. A.; Navarro, R.; Núñez Peral, A.; Royo, M.; Sánchez, A.; Silva-López, M.; Vera, I.; Villanueva, J.; Fourmond, J. -J.; de Galarreta, C. Ruiz; Bouzit, M.; Hervier, V.; Le Clec'h, J. C.; Szwec, N.; Chaigneau, M.; Buttice, V.; Dominguez-Tagle, C.; Philippon, A.; Boumier, P.; Le Cocguen, R.; Baranjuk, G.; Bell, A.; Berkefeld, Th.; Baumgartner, J.; Heidecke, F.; Maue, T.; Nakai, E.; Scheiffelen, T.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Blanco Rodríguez, J.; Domingo, V.; Ferreres Sabater, A.; Gasent Blesa, J. L.; Rodríguez Martínez, P.; Osorno Caudel, D.; Bosch, J.; Casas, A.; Carmona, M.; Herms, A.; Roma, D.; Alonso, G.; Gómez-Sanjuan, A.; Piqueras, J.; Torralbo, I.; Fiethe, B.; Guan, Y.; Lange, T.; Michel, H.; Bonet, J. A.; Fahmy, S.; Müller, D.; Zouganelis, I. Bibcode: 2020A&A...642A..11S Altcode: 2019arXiv190311061S
Aims: This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, while hosting the potential of a rich return in further science.
Methods: SO/PHI measures the Zeeman effect and the Doppler shift in the Fe I 617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO3 Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders. The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope, covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope, can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line.
Results: SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission's highly elliptical orbit. Title: Subpixel real-time jitter detection algorithm and implementation for polarimetric and helioseismic imager Authors: Roma, David; Carmona, Manuel; Bosch, Jose; Casas, Albert; Herms, Atila; Lopez, Manel; Ruiz, Oscar; Sabater, Josep; Berkefeld, Thomas; Maue, Thorsten; Nakai, Eiji; Schmidt, Wolfgang; Soltau, Dirk; Volkmer, Reiner; Gomez, Jose M. Bibcode: 2019JATIS...5c9003R Altcode: The polarimetric and helioseismic imager instrument for the Solar Orbiter mission from the European Space Agency requires a high stability while capturing images, specially for the polarimetric ones. For this reason, an image stabilization system has been included in the instrument. It uses global motion estimation techniques to estimate the jitter in real time with subpixel resolution. Due to instrument requirements, the algorithm has to be implemented in a Xilinx Virtex-4QV field programmable gate array. The algorithm includes a 2-D paraboloid interpolation algorithm based on 2-D bisection. We describe the algorithm implementation and the tests that have been made to verify its performance. The jitter estimation has a mean error of 125 pixel of the correlation tracking camera. The paraboloid interpolation algorithm provides also better results in terms of resources and time required for the calculation (at least a 20% improvement in both cases) than those based on direct calculation. 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: 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: Use of adaptive optics in ground stations for high data rate satellite-to-ground links Authors: Fischer, E.; Berkefeld, Th.; Feriencik, Mikael; Feriencik, Marco; Kaltenbach, V.; Soltau, D.; Adolph, P.; Czichy, R.; Kunde, J.; Heine, F.; Saucke, K.; Meyer, R.; Richter, I.; Sodnik, Z. Bibcode: 2017SPIE10562E..3LF Altcode: Over the past years we have successfully applied adaptive optics (AO) in some optical ground stations (OGS) to improve the signal-to-noise ratio of satellite to ground laser communications. 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: The adaptive optics system of the 1.5m GREGOR solar telescope: four years of operation Authors: Berkefeld, Thomas; Schmidt, Dirk; Soltau, Dirk; Heidecke, Frank; Fischer, Andreas Bibcode: 2016SPIE.9909E..24B Altcode: We present the properties of the adaptive optics (AO) system of the German 1.5m solar telescope GREGOR, located on the island of Tenerife, Spain. The conventional AO system uses a correlating Shack-Hartmann-Sensor with a 92mm subaperture size and a 256-actuator stacked-piezo deformable mirror (DM). AO performance results and practical experience based on the last four years of operation are presented. A recently installed second wavefront sensor with exchangeable lenslets / subaperture sizes in combination with an EM-CCD camera is used for low light observations such as polarimetric measurements of the solar system planets. Further developments include algorithmic improvements, the use of the night-time sensor for solar (off-limb) observations and solar MCAO. 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: 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: Recent results and future DMs for astronomy and for space applications at CILAS Authors: Sinquin, Jean-Christophe; Bastard, Arnaud; Beaufort, Emmanuel; Berkefeld, Thomas; Cadiergues, Laurent; Costes, Vincent; Cousty, Raphaël.; Dekhtiar, Charles; Di Gesu, Frédéric; Gilbert, Xavier; Grèzes-Besset, Catherine; Groeninck, Denis; Hartung, Markus; Krol, Hélène; Moreau, Aurélien; Morin, Pierre; Pagès, Hubert; Palomo, Richard; Scharmer, Göran; Soltau, Dirk; Véran, Jean-Pierre Bibcode: 2014SPIE.9148E..0GS Altcode: We present recent experimental results obtained with CILAS deformable mirrors (DMs) or demonstration prototypes in solar and night-time astronomy (with ground-based telescopes) as well as observation of the Earth (with space telescopes). These important results have been reached thanks to CILAS technology range composed of monomorph and piezostack deformable mirrors, drivers and optical coatings. For instance, the monomorph technology, due to a simple architecture can offer a very good reliability for space applications. It can be used for closed or open loop correction of the primary mirror deformation (thermal and polishing aberrations, absence of gravity). It can also allow a real-time correction of wavefront aberrations introduced by the atmosphere up to relatively high spatial and temporal frequencies for ground-based telescopes. The piezostack technology is useful for very high order correction at high frequency and under relatively low operational temperature (down to -30°C), which is required for future Extremely Large Telescopes (ELTs). This wide range of applications is exposed through recent examples of DMs performances in operation and results obtained with breadboards, allowing promising DMs for future needs. Title: First successful deployment of the ZIMPOL-3 system at the GREGOR telescope Authors: Ramelli, Renzo; Gisler, Daniel; Bianda, Michele; Bello González, Nazaret; Berdyugina, Svetlana; Soltau, Dirk Bibcode: 2014SPIE.9147E..3GR Altcode: Since several years the Zurich Imaging polarimeter (ZIMPOL) system is successfully used as a high sensitivity polarimeter. The polarimeter system, which is mainly based on a fast modulator and a special demodulating camera with a masked CCD, has been continuously improved. The third version of the system (ZIMPOL-3) is routinely used at IRSOL, Locarno. The fast modulation allows to "freeze" intensity variations due to seeing, and to achieve a polarimetric sensitivity below 10-5 if the photon statistics is large enough. In October 2013 the ZIMPOL system has been brought and installed for the first time at the GREGOR telescope in Tenerife for a spectropolarimetric observing campaign. There, the system configuration took advantage from the calibration unit installed at the primary focus of the GREGOR telescope, while the analyzer was inserted in the optical path just before the spectrograph slit after several folding mirrors. This setup has been tested successfully by the authors for the first time in this occasion. Title: EST: the largest and most sensitive spectropolarimeter Authors: Collados, M.; Bettonvil, F.; Cavaller, L.; Ermolli, I.; Gelly, B.; Pérez, A. : Socas-Navarro, H.; Soltau, D.; Volkmer, R.; EST Team Bibcode: 2013hsa7.conf..808C Altcode: Magnetic field plays a crucial role to understand most phenomena happening in the solar atmosphere. Sunspots, flares, prominences, coronal mass ejections are well known examples of its interaction with the solar plasma. To study the properties of this interaction, one needs to analyze the imprint it leaves in the radiation through the polarization induced in spectral lines, via the Zeeman and Hanle effects. Outside sunspots, the polarization degree of the emitted light is usually well below one part in one thousand, which requires sophisticated techniques to measure it accurately. To further complicate the situation, telescopes use mirrors and these introduce undesired polarization which is two or three orders of magnitude larger than that caused by the magnetic field of solar structures. For this reason, present telescopes doing polarimetry require an adequate modelling to correct the measured data from these spurious effects. In addition, most of the magnetic field interactions with the plasma take place at small scales. The best achievable angular resolution is mandatory to adequately study magnetic phenomena. The European solar Telescope (EST) has been defined to overcome these difficulties. Here, some aspects of the design are described. Title: The European Solar Telescope Authors: Collados, M.; Bettonvil, F.; Cavaller, L.; Ermolli, I.; Gelly, B.; Pérez, A.; Socas-Navarro, H.; Soltau, D.; Volkmer, R.; EST Team Bibcode: 2013MmSAI..84..379C Altcode: The European Solar Telescope (EST) is a project to design, build and operate an European Solar 4-meter class telescope to be located in the Canary Islands, with the participation of institutions from fifteen European countries gathered around the consortium EAST (European Association for Solar Telescopes). The project main objective up to the present has been the development of the conceptual design study (DS) of a large aperture Solar Telescope. The study has demonstrated the scientific, technical and financial feasibility of EST. The DS has been possible thanks to the co-financing allocated specifically by the EU and the combined efforts of all the participant institutions. Different existing alternatives have been analysed for all telescope systems and subsystems, and decisions have been taken on the ones that are most compatible with the scientific goals and the technical strategies. The present status of some subsystems is reviewed in this paper. 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: 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 polarimetric calibration unit Authors: Hofmann, A.; Arlt, K.; Balthasar, H.; Bauer, S. M.; Bittner, W.; Paschke, J.; Popow, E.; Rendtel, J.; Soltau, D.; Waldmann, T. Bibcode: 2012AN....333..854H Altcode: The new Solar telescope GREGOR is designed to observe small-scale dynamic magnetic structures below a size of 70 km on the Sun with high spectral resolution and polarimetric accuracy. For this purpose, the polarimetric concept of GREGOR is based on a combination of post-focus polarimeters with pre-focus equipment for high precision calibration. The Leibniz-Institute for Astrophysics Potsdam developed the GREGOR calibration unit which is an integral part of the telescope. We give an overview of the function and design of the calibration unit and present the results of extensive testing series done in the Solar Observatory ``Einsteinturm'' and at GREGOR. 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: 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: Preparing the GREGOR solar telescope for night-time use: Deriving a pointing model Authors: Granzer, T.; Halbgewachs, C.; Volkmer, R.; Soltau, D. Bibcode: 2012AN....333..823G Altcode: We report on the results of a dedicated campaign to derive a pointing model for the GREGOR solar telescope which took place in December 2011. Two main goals were in the focus of this campaign: first to prove the aptness of the GREGOR solar telescope for night-time, unattended operations and second to derive some qualitative measure of the amount of misalignment in the optical and mechanical parts of the telescope. In the final version, a root-mean-square deviation (RMSD) of 1.6 arcsec for the azimuth model and an RMSD of 2.3 arcsec in the elevation model could be achieved. Title: Image stabilisation system of the photospheric and helioseismic imager Authors: Volkmer, R.; Bosch, J.; Feger, B.; Gomez, J. M.; Heidecke, F.; Schmidt, W.; Scheiffelen, T.; Sigwarth, M.; Soltau, D. Bibcode: 2012SPIE.8442E..4PV Altcode: The Photospheric and Helioseismic imager (PHI) on board of the ESA mission Solar Orbiter, to be launched in 2017, will provide measurements with high polarimetric accuracy of the photospheric solar magnetic field at high solar latitudes. The needed pointing precision requires an image stabilisation (ISS) to compensate for spacecraft jitter. The image stabilisation system works as a correlation tracker with a high-speed camera and a fast steerable mirror. The optomechanical and electronic design of the system will be presented. Title: Polarimetry with GREGOR Authors: Balthasar, H.; Bello González, N.; Collados, M.; Denker, C.; Feller, A.; Hofmann, A.; Lagg, A.; Nagaruju, L.; Puschmann, K. G.; Soltau, D.; Volkmer, R. Bibcode: 2011ASPC..437..351B Altcode: A brief description of the new 1.5-meter solar telescope GREGOR located at the Observatorio del Teide in Tenerife will be given. GREGOR will provide a spatial resolution of about 75 km on the Sun, and with its light collecting capability we will be able to study the development of small magnetic features with high cadence. From the beginning, it will be equipped with the GREGOR Fabry-Pérot Interferometer (GFPI) for the visible spectral range and with a GRating Infrared Spectrograph (GRIS). Both postfocus instruments can be combined with a polarimeter, and in both cases the light is modulated by two ferro-electric liquid crystals. A calibration unit can be inserted to determine the instrumental polarization. Because of the altazimuthal mount, time-dependent rotation of the polarimetric reference plane is introduced, and we have to develop a polarization model of the telescope. Measurements to verify this model are in preparation. Title: The Wave-Front Correction System for the Sunrise Balloon-Borne Solar Observatory Authors: Berkefeld, T.; Schmidt, W.; Soltau, D.; Bell, A.; Doerr, H. P.; Feger, B.; Friedlein, R.; Gerber, K.; Heidecke, F.; Kentischer, T.; v. d. Lühe, O.; Sigwarth, M.; Wälde, E.; Barthol, P.; Deutsch, W.; Gandorfer, A.; Germerott, D.; Grauf, B.; Meller, R.; Álvarez-Herrero, A.; Knölker, M.; Martínez Pillet, V.; Solanki, S. K.; Title, A. M. Bibcode: 2011SoPh..268..103B Altcode: 2010SoPh..tmp..236B; 2010arXiv1009.3196B This paper describes the wave-front correction system developed for the Sunrise balloon telescope, and it provides information about its in-flight performance. For the correction of low-order aberrations, a Correlating Wave-Front Sensor (CWS) was used. It consisted of a six-element Shack - Hartmann wave-front sensor (WFS), a fast tip-tilt mirror for the compensation of image motion, and an active telescope secondary mirror for focus correction. The CWS delivered a stabilized image with a precision of 0.04 arcsec (rms), whenever the coarse pointing was better than ± 45 arcsec peak-to-peak. The automatic focus adjustment maintained a focus stability of 0.01 waves in the focal plane of the CWS. During the 5.5 day flight, good image quality and stability were achieved during 33 hours, containing 45 sequences, which lasted between 10 and 45 min. Title: The Filter Imager SuFI and the Image Stabilization and Light Distribution System ISLiD of the Sunrise Balloon-Borne Observatory: Instrument Description Authors: Gandorfer, A.; Grauf, B.; Barthol, P.; Riethmüller, T. L.; Solanki, S. K.; Chares, B.; Deutsch, W.; Ebert, S.; Feller, A.; Germerott, D.; Heerlein, K.; Heinrichs, J.; Hirche, D.; Hirzberger, J.; Kolleck, M.; Meller, R.; Müller, R.; Schäfer, R.; Tomasch, G.; Knölker, M.; Martínez Pillet, V.; Bonet, J. A.; Schmidt, W.; Berkefeld, T.; Feger, B.; Heidecke, F.; Soltau, D.; Tischenberg, A.; Fischer, A.; Title, A.; Anwand, H.; Schmidt, E. Bibcode: 2011SoPh..268...35G Altcode: 2010SoPh..tmp..176G; 2010arXiv1009.1037G We describe the design of the Sunrise Filter Imager (SuFI) and the Image Stabilization and Light Distribution (ISLiD) unit onboard the Sunrise balloon borne solar observatory. This contribution provides the necessary information which is relevant to understand the instruments' working principles, the relevant technical data, and the necessary information about calibration issues directly related to the science data. 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: Current concept for the 4m European Solar Telescope (EST) optical design Authors: Sánchez Capuchino, J.; Collados, M.; Soltau, D.; López, R.; Rasilla, J. L.; Gelly, B. Bibcode: 2010SPIE.7652E..0SS Altcode: 2010SPIE.7652E..26S The European Solar Telescope (EST) is a 4-m class solar telescope to be located in the Canary Islands which is currently in its conceptual design study. EST is a pan-european project (with 29 partners, plus 7 collaborating institutions, from 14 countries) promoted by the European Association for Solar Telescopes (EAST). In the current concept, the main telescope and its transfer optics assemblies 14 mirrors to provide a Science Coudé Focus with an F/50 telecentric beam. It is diffraction-limited in a FOV of 1 arcmin with an unvignetted FOV of 2'x2'. The whole system is being optimized in throughput for several instruments observing simultaneously in a spectral range from 0.39 μm to 2.3 μm. Its innovative concept integrates an optical transfer stage assembling multiconjugated adaptive optics with optical field de-rotation and with a perfect balance of the whole system in terms of polarization being also time and wavelength invariant. Title: Wavefront sensing and wavefront reconstruction for the 4m European Solar Telescope EST Authors: Berkefeld, Thomas; Soltau, Dirk; del Moro, Dario; Löfdahl, Mats Bibcode: 2010SPIE.7736E..2JB Altcode: 2010SPIE.7736E..85B We give an overview of the Adaptive Optics (AO) and Multi-conjugate Adaptive Optics (MCAO) system of the planned 4m European Solar Telescope (EST). The parameter space and the problems of solar MCAO working in the visible are explained. The wavefront reconstruction schemes presently being considered are explained. First estimates of the expected MCAO performance for varying parameter sets are given. 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: European Solar Telescope: project status Authors: Collados, M.; Bettonvil, F.; Cavaller, L.; Ermolli, I.; Gelly, B.; Grivel-Gelly, C.; Pérez, A.; Socas-Navarro, H.; Soltau, D.; Volkmer, R. Bibcode: 2010SPIE.7733E..0HC Altcode: 2010SPIE.7733E..15C The European Solar Telescope is a project for a 4-meter class telescope to be located in the Canary Islands. EST is promoted by the European Association for Solar Telescopes (EAST). This is a consortium formed by a number of research organizations from fifteen European countries (Austria, Croatia, Czech Republic, France, Germany, Hungary, Italy, the Netherlands, Norway, Poland, Slovak Republic, Spain, Sweden, Switzerland, and United Kingdom). EST will specialize in high spatial and temporal resolution using diverse instruments that can efficiently produce two-dimensional spectropolarimetric information of the thermal, dynamic and magnetic properties of the plasma over many scale heights in the solar atmosphere. In this contribution, the status of the development of the Design Study of EST is presented, emphasizing the most important aspects of the optical design, mechanical structure, AO and MCAO systems for wavefront correction, instruments and polarization analysis. Title: Solar multiconjugate adaptive optics at the Dunn Solar Telescope Authors: Rimmele, T. R.; Woeger, F.; Marino, J.; Richards, K.; Hegwer, S.; Berkefeld, T.; Soltau, D.; Schmidt, D.; Waldmann, T. Bibcode: 2010SPIE.7736E..31R Altcode: 2010SPIE.7736E.101R Solar observations are performed over an extended field of view and the isoplanatic patch over which conventional adaptive optics (AO) provides diffraction limited resolution is a severe limitation. The development of multi-conjugate adaptive optics (MCAO) for the next generation large aperture solar telescopes is thus a top priority. The Sun is an ideal object for the development of MCAO since solar structure provides multiple "guide stars" in any desired configuration. At the Dunn Solar Telescope (DST) we implemented a dedicated MCAO bench with the goal of developing wellcharacterized, operational MCAO. The MCAO system uses two deformable mirrors conjugated to the telescope entrance pupil and a layer in the upper atmosphere, respectively. The high altitude deformable mirror can be placed at conjugates ranging from 2km to 10km altitude. We have successfully and stably locked the MCAO system on solar granulation and demonstrated the MCAO system's ability to significantly extend the corrected field of view. We present results derived from analysis of imagery taken simultaneously with conventional AO and MCAO. We also present first results from solar Ground Layer AO (GLAO) experiments. Title: Site-seeing measurements for the European Solar Telescope Authors: Berkefeld, Th.; Bettonvil, F.; Collados, M.; López, R.; Martín, Y.; Peñate, J.; Pérez, A.; Scharmer, G. B.; Sliepen, G.; Soltau, D.; Waldmann, T. A.; van Werkhoven, T. Bibcode: 2010SPIE.7733E..4IB Altcode: 2010SPIE.7733E.141B Seeing measurements are crucial for the optimum design of (multi-conjugate) adaptive optics systems operating at solar telescopes. For the design study of the 4-meter European Solar Telescope, to be located in the Canary Islands, several instruments have been constructed and operated, at the Observatorio del Roque de los Muchachos (La Palma) and at the Observatorio del Teide (Tenerife), to measure the properties of the ground layer and medium-high altitude turbulence. Several units of short (42.34 cm) and two long (323.06 cm) scintillometer bars are, or are to be, installed at both observatories. In addition to them, two wide-field wavefront sensors will be attached to the optical beams of the Swedish tower, on La Palma, and of the German VTT, on Tenerife, simultaneously used with the normal operation of the telescopes. These wavefront sensors are of Shack-Hartmann type with ~1 arcminute field of view. In this contribution, the instruments setup and their performance are described. Title: Adaptive optics for satellite-to-ground laser communication at the 1m Telescope of the ESA Optical Ground Station, Tenerife, Spain Authors: Berkefeld, Thomas; Soltau, Dirk; Czichy, Reinhard; Fischer, Edgar; Wandernoth, Bernhard; Sodnik, Zoran Bibcode: 2010SPIE.7736E..4CB Altcode: 2010SPIE.7736E.146B We present the setup including optics and components, the reconstruction scheme and performance estimations of an Adaptive Optics (AO) system implemented at the 1m telescope of the ESA Optical Ground Station (OGS), Observatorio del Teide, Tenerife. The system will be used to improve the signal-to-noise ratio of satellite to ground laser communications. It operates with coherent laser communication systems at 1064nm. The wavefront sensor is an 88-element Shack-Hartmann-sensor (11 subapertures across the pupil), matched to a 12×12 actuator "Multi-DM" membrane deformable mirror (DM). The system is able to remove a large part of the turbulence-induced and static wavefront errors by using more than 90 degrees of freedom ("modes"). Due to a special high speed infrared camera, the control loop can run at speeds up to 20 kHz, achieving a 0db bandwidth of about 500Hz, depending on the received laser power. Title: Adaptive optics and MCAO for the 4-m European Solar Telescope EST Authors: Soltau, D.; Berkefeld, T.; Sánchez Capuchino, J.; Collados Vera, M.; Del Moro, D.; Löfdahl, M.; Scharmer, G. Bibcode: 2010SPIE.7736E..0US Altcode: 2010SPIE.7736E..27S A consortium of more than 20 European solar physics institution from 15 different countries is conducting a design study for a 4 m class solar telescope which shall be situated at the Canary Islands. In this paper we introduce the AO and MCAO design concept for EST. A ground layer deformable mirror is combined with an arrangement of four deformable layer mirrors. A combination of Shack-Hartmann wave front sensors with wide and narrow fields of view is used to control the system and to achieve a corrected field of view of one arcmin. Title: Current concept for the 4m European Solar Telescope (EST) optical design Authors: Sánchez-Capuchino, J.; Collados, M.; Soltau, D.; López, R.; Rasilla, J. L.; Gelly, B. Bibcode: 2010SPIE.7733E..36S Altcode: 2010SPIE.7733E..99S The European Solar Telescope (EST) is a pan-european project (with 29 partners, plus 7 collaborating institutions, from 14 countries) for the conceptual design study of a 4-meter class solar telescope promoted by the European Association for Solar Telescopes (EAST) to be located in the Canary Islands. The telescope, in the conceptual study, provides a Coudé focus with an F/50 telecentric beam. It is diffraction-limited in a FOV of 1 arcmin and it will be optimized in throughput for several instruments observing simultaneously in a spectral range from 0.39 μm to 2.3 μm. Its innovative concept integrates an optical transfer stage assembling multiconjugated adaptive optics with optical field de-rotation and with a perfect balance of the whole system in terms of polarization being time and wavelength invariant. Title: NLST: the Indian National Large Solar Telescope Authors: Hasan, S. S.; Soltau, D.; Kärcher, H.; Süss, M.; Berkefeld, T. Bibcode: 2010SPIE.7733E..0IH Altcode: 2010SPIE.7733E..16H India is planning a new solar telescope with an aperture of 2-m for carrying out high resolution studies of the Sun. Site characterization is underway at high altitude locations in the Himalayan mountains. A detailed concept design for NLST (National Large Solar Telescope) has been completed. The optical design of the telescope is optimized for high optical throughput and uses a minimum number of optical elements. A high order AO system is integrated part of the design that works with a modest Fried's parameter of 7-cm to give diffraction limited performance. The telescope will be equipped with a suite of post-focus instruments including a high resolution spectrograph and a polarimeter. NLST will also be used for carrying out stellar observations during the night. The mechanical design of the telescope, building, and the innovative dome is optimized to take advantage of the natural air flush which will help to keep the open telescope in temperature equilibrium. After its completion (planned for 2014), NLST will fill a gap in longitude between the major solar facilities in USA and Europe, and it will be for years the largest solar telescope in the world Title: Flight control software for the wave-front sensor of SUNRISE 1m balloon telescope Authors: Bell, Alexander; Barthol, Peter; Berkefeld, Thomas; Feger, Bernhard; Gandorfer, Achim M.; Heidecke, Frank; Knoelker, Michael; Martinez Pillet, Valentin; Schmidt, Wolfgang; Sigwarth, Michael; Solanki, Sami K.; Soltau, Dirk; Title, Alan M. Bibcode: 2010SPIE.7740E..03B Altcode: 2010SPIE.7740E...2B This paper describes the flight control software of the wave-front correction system that flew on the 2009 science flight of the Sunrise balloon telescope. The software discussed here allowed fully automated operations of the wave-front sensor, communications with the adaptive optics sub-system, the pointing system, the instrument control unit and the main telescope controller. The software was developed using modern object oriented analysis and design techniques, and consists of roughly 13.000 lines of C++ code not counting code written for the on-board communication layer. The software operated error free during the 5.5 day flight. Title: European Solar Telescope: Progress status Authors: Collados, M.; Bettonvil, F.; Cavaller, L.; Ermolli, I.; Gelly, B.; Pérez, A.; Socas-Navarro, H.; Soltau, D.; Volkmer, R.; EST Team Bibcode: 2010AN....331..615C Altcode: In this paper, the present status of the development of the design of the European Solar Telescope is described. The telescope is devised to have the best possible angular resolution and polarimetric performance, maximizing the throughput of the whole system. To that aim, adaptive optics and multi-conjugate adaptive optics are integrated in the optical path. The system will have the possibility to correct for the diurnal variation of the distance to the turbulence layers, by using several deformable mirrors, conjugated at different heights. The present optical design of the telescope distributes the optical elements along the optical path in such a way that the instrumental polarization induced by the telescope is minimized and independent of the solar elevation and azimuth. This property represents a large advantage for polarimetric measurements. The ensemble of instruments that are planned is also presented. Title: NLST: India's National Large Solar Telescope Authors: Hasan, S. S.; Soltau, D.; Kärcher, H.; Süß, M.; Berkefeld, T. Bibcode: 2010AN....331..628H Altcode: This article introduces the new Indian 2 m telescope which has been designed by MT Mechatronics in a detailed conceptual design study for the Indian Institute of Astrophysics, Bangalore. We describe the background of the project and the science goals which shall be addressed with this telescope. NLST is a solar telescope with high optical throughput and will be equipped with an integrated Adaptive Optics system. It is optimized for a site with the kind of seeing and wind conditions as they are expected at a lake site in the Himalayan mountains. The telescope can also be used for certain night time applications. We also give the scientific rationale for this class of telescope. 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: EST Adaptive optics performance estimations Authors: Berkefeld, T.; Soltau, D. Bibcode: 2010AN....331..640B Altcode: We give a short overview of the Adaptive Optics (AO) and Multi-conjugate Adaptive Optics (MCAO) system of the planned 4 m European Solar Telescope (EST). The optimization process of the AO / MCAO parameters is shown, including the parameters and layout of the Shack-Hartmann wavefront sensor setup and the DMs. We show the expected performance of the AO and MCAO system. 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: Concepts for Multi-wavelength Observations with the 1.5 m Solar Telescope GREGOR Authors: Schmidt, W.; Beck, C.; Denker, C.; Soltau, D.; Volkmer, R. Bibcode: 2008ESPM...12.2.72S Altcode: The initial instrumentation of the 1.5 m solar telescope GREGOR on Tenerife will include a grating spectrograph with a spectro-polarimeter for the near infrared and a dual Fabry-Perot filter spectro-polarimeter for the "red" wavelength range from 530 to 860 nm. Both instruments are based on their predecessors that are presently operated at the Vacuum-Tower Telescope on Tenerife. In order to fully exploit the scientific capability of GREGOR, simultaneous multi-wavelength observations are necessary. Such observations will allow studying the coupling of different layers of the solar atmosphere. Simultaneous measurements of the magnetic field, using different spectral lines, increase the sensitivity and the accuracy of such measurements.

We present concepts for multi-wavelength observations with GREGOR based on the first-light instruments. As a first step, fast large-format CCD cameras will be used in the slit-jaw imaging system of the spectrograph or in an additional medium-band imaging channel for post-facto image restoration combined with the spectro-polarimeter. These observations with high spatial resolution would provide the context for the spectrally resolved data. Beyond simple time-share concepts, the different characteristics of imaging and spectrograph-based instruments require more sophisticated approaches, which should neither sacrifice adaptive optics performance nor diminish polarimetric efficiency and accuracy. Here, we will discuss a beam splitter option and variants of scanning spectrograph systems. Finally, we introduce a concept for a second-generation instrument the "Blue Imaging Solar Spectrometer" (BLISS), a dual Fabry-Perot interferometer to explore the blue solar spectrum (from 380 to 530 nm) in tandem with the "red" instrument. 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: 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: SUNRISE: high resolution UV/VIS observations of the Sun from the stratosphere Authors: Gandorfer, A. M.; Solanki, S. K.; Barthol, P.; Lites, B. W.; Martínez Pillet, V.; Schmidt, W.; Soltau, D.; Title, A. M. Bibcode: 2006SPIE.6267E..0SG Altcode: 2006SPIE.6267E..25G SUNRISE is an international project for the development, construction, and operation of a balloon-borne solar telescope with an aperture of 1 m, working in the UV/VIS spectral domain. The main scientific goal of SUNRISE is to understand the structure and dynamics of the magnetic field in the atmosphere of the Sun. SUNRISE will provide near diffraction-limited images of the photosphere and chromosphere with an unpredecented resolution down to 35 km on the solar surface at wavelengths around 220 nm. The focal-plane instrumentation consists of a polarization sensitive spectrograph, a Fabry-Perot filter magnetograph, and a phase-diverse filter imager working in the near UV. The first stratospheric long-duration balloon flight of SUNRISE is planned in Summer 2009 from the swedish ESRANGE station. SUNRISE is a joint project of the german Max-Planck-Institut fur Sonnensystemforschung (MPS), Katlenburg-Lindau, with the Kiepenheuer-Institut fur Sonnenphysik (KIS), Freiburg, Germany, the High-Altitude Observatory (HAO), Boulder, USA, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, USA, and the spanish IMaX consortium. In this paper we will present an actual update on the mission and give a brief description of its scientific and technological aspects. Title: Auto alignment and image tracking system for the SUNRISE telescope Authors: Schmidt, W.; Berkefeld, T.; Feger, B.; Friedlein, R.; Gerber, K.; Heidecke, F.; Kentischer, T.; Lühe, O. v. d.; Sigwarth, M.; Soltau, D.; Wälde, E. Bibcode: 2006SPIE.6274E..0HS Altcode: 2006SPIE.6274E..14S The 1m balloon-borne solar telescope Sunrise will be equipped with a wave-front sensing system for automatic in-flight focusing and alignment of the telescope and for high-precision image tracking. A six-element wavefront sensor measures low order aberrations of the telescope, including defocus and coma. The correction is achieved by moving the focusing mirror and the telescope secondary, respectively, in a closed-loop circuit. The same system measures image motion. The instrument requirements for the tracking are a dynamical range of about 30 Hz and a precision of about 0.005 arcs in the sky. The image motion signal feeds a closed-loop control system that drives both the tip-tilt mirror assembly and the mirrors that are needed for focusing and alignment. The tip-tilt unit is a dual-stage system, built at the Kiepenheuer-Insitut, consisting of a slow component with a large range of about 60 arcs and a fast component with a short range and high bandwidth. A breadboard-version of the Correlating Wavefront Sensor has been successfully tested at the German Vacuum Tower Telescope on Tenerife in summer of 2005. A closed-loop bandwidth of 80 Hz was measured for the tracking system. The wave-front sensor detected image aberrations pre-set by the telescope's adaptive optics system with the required accuracy. Sunrise will be flown in long duration stratospheric balloon flights, with a first scientific flight in 2009. Title: GREGOR AO as a tool for telescope commissioning Authors: Soltau, D.; Berkefeld, Th.; Volkmer, R. Bibcode: 2006SPIE.6267E..11S Altcode: 2006SPIE.6267E..34S The new German solar 1.5 m telescope (GREGOR) will be equipped with an adaptive optic system. GREGOR has a relatively complicated optical scheme with small tolerances. We therefore have to expect certain aberrations due to misalignments and mechanical/optical imperfections. This is why the AO will play an important role as an auxiliary tool for telescope alignment from the very beginning of the commissioning phase. The paper will cover the alignment strategies taking advantage of the AO system. 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: 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: The ATST Site Survey Authors: Hill, F.; Beckers, J.; Brandt, P.; Briggs, J. W.; Brown, T.; Brown, W.; Collados, M.; Denker, C.; Fletcher, S.; Hegwer, S.; Horst, T.; Komsa, M.; Kuhn, J.; Lecinski, A.; Lin, H.; Oncley, S.; Penn, M.; Radick, R.; Rimmele, T.; Socas-Navarro, H.; Soltau, D.; Streander, K. Bibcode: 2005AGUSMSP34A..04H Altcode: The Advanced Technology Solar Telescope (ATST) will be the world's largest aperture solar telescope, and is being designed for high resolution, IR, and coronal research. It must be located at a site that maximizes the scientific return of this substantial investment. We present the instrumentation, analysis and results of the ATST site survey. Two instrumentation sets were deployed at each of six sites to measure seeing as a function of height, and sky brightness as a function of wavelength and off-limb position. Analysis software was developed to estimate the structure function Cn2 as a function of height near the ground, and the results were verified by comparison with in-situ measurements. Additional software was developed to estimate the sky brightness. The statistics of the conditions at the sites were corrected for observing habits and the annualized hours of specific observing conditions were estimated. These results were used to identify three excellent sites suitable to host the ATST: Haleakala, Big Bear and La Palma. Among them, Haleakala is proposed as the optimal location of the ATST, La Palma and Big Bear being viable alternative sites. Title: Penumbral Line Asymmetries Using KAOS Authors: Soltau, Dirk; Berkefeld, Thomas; Schlichenmaier, Rolf; Tritschler, Alexandra; Rubio, Luis Ramon Bellot Bibcode: 2005sao..conf..129S Altcode: We analyse and interpret the line asymmetries in a sunspot penumbra at a heliocentric angle of 23°. The data, acquired with TESOS [1] and KAOS [2] at the VTT, is of high spatial (0.5") and high spectral ( λ/∆λ = 250 000) resolution. We observed in Fe I 557.6 nm (g=0) [3]. 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: Detection of neutral sodium above Mercury during the transit on 2003 May 7 Authors: Schleicher, H.; Wiedemann, G.; Wöhl, H.; Berkefeld, T.; Soltau, D. Bibcode: 2004A&A...425.1119S Altcode: The extent of the exosphere of Mercury above the planet's limb could for the first time be observed by detecting an excess absorption in the solar sodium line D_2 during the transit of Mercury across the solar disk on 2003 May 7. The

observations were performed with a 2d Fabry-Perot spectrograph of the Vacuum Tower Telescope at Izaña, Tenerife. The absorption excess, blue-shifted by 13 pm relative to the solar line, is mainly concentrated near the polar regions. There,

the absorption excess can be traced up to ≈700 km above the limb. Between the two polar regions, along the eastern limb, a weaker absorption excess can be seen. A possible streamer-like feature stretches more than 2000 km above the northern region. Assuming the density to decrease exponentially with height, we derive for the polar maxima vertical column densities of 3 × 1010 cm-2, volume densities at the surface of 2.5 × 103 cm-3, and a density scale height of 150 km. Title: Latest Results from the ATST Site Survey Authors: Hill, F.; Collados, M.; Navarro, H.; Beckers, J.; Brandt, P.; Briggs, J.; Brown, T.; Denker, C.; Hegwer, S.; Horst, T.; Komsa, M.; Kuhn, J.; Lin, H.; Oncley, S.; Penn, M.; Rimmele, T.; Soltau, D.; Streander, K. Bibcode: 2004AAS...204.6909H Altcode: 2004BAAS...36..795H We present the latest results and current status of the site survey portion of the Advanced Technology Solar Telescope (ATST) project. The ATST will provide high resolution solar data in the visible and IR. The site is a major factor determining the performance of the telescope. The most critical site characteristics are the statistics of daytime seeing quality and sky clarity. These conditions are being measured by a suite of instruments at three sites (Big Bear, Haleakala, La Palma). These sites were chosen from a set of six that have been tested starting in November 2001. The instrumentation includes a solar differential image motion monitor, an array of scintillometers, a miniature coronagraph, a dust monitor, and a weather station. The analysis of the data provides an estimate of the seeing as a function of height near the ground. We will present the latest results of the analysis of the survey data set. Title: Solar constraints on new couplings between electromagnetism and gravity Authors: Solanki, S. K.; Preuss, O.; Haugan, M. P.; Gandorfer, A.; Povel, H. P.; Steiner, P.; Stucki, K.; Bernasconi, P. N.; Soltau, D. Bibcode: 2004PhRvD..69f2001S Altcode: 2004gr.qc.....2055S The unification of quantum field theory and general relativity is a fundamental goal of modern physics. In many cases, theoretical efforts to achieve this goal introduce auxiliary gravitational fields, ones in addition to the familiar symmetric second-rank tensor potential of general relativity, and lead to nonmetric theories because of direct couplings between these auxiliary fields and matter. Here, we consider an example of a metric-affine gauge theory of gravity in which torsion couples nonminimally to the electromagnetic field. This coupling causes a phase difference to accumulate between different polarization states of light as they propagate through the metric-affine gravitational field. Solar spectropolarimetric observations are reported and used to set strong constraints on the relevant coupling constant k: k2<(2.5 km)2. Title: Exosphere of Mercury seen as additional absorption in the Na D2 line during the Transit on 2003 May 7. Authors: Schleicher, H.; Wiedemann, G.; Woehl, H.; Berkefeld, T.; Soltau, D. Bibcode: 2004ANS...325...81S Altcode: 2004ANS...325a..81S; 2004ANS...325..P05S 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: 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: Comparison of Methods for Fried Parameter Estimation Authors: Wöger, Friedrich; Berkefeld, Thomas; Soltau, Dirk Bibcode: 2003ANS...324R..22W Altcode: 2003ANS...324..C03W No abstract at ADS Title: Merkur vor der Sonne. Eine Fisternis der besonderen Art Authors: Mattig, Wolfgang; Soltau, Dirk Bibcode: 2003S&W....42d..66M Altcode: 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: 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: KAOS: Adaptive optics system for the Vacuum Tower Telescope at Teide Observatory Authors: van der Luehe, Oskar; Soltau, Dirk; Berkefeld, Thomas; Schelenz, Thomas Bibcode: 2003SPIE.4853..187V Altcode: We are completing the integration of a solar adaptive optics system KAOS at the 70 cm diameter Vacuum Tower telescope (VTT) on Tenerife. The system is capable to compensate some 30 modes of wavefront aberration with closed-loop bandwidth of about 100 Hz anywhere on the solar disk. We describe the design goals, the main characteristics of KAOS and present a first demonstration of its performance. 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: 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: 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: 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: MCAO at the German solar vacuum telescope Authors: Berkerfled, Thomas; Soltau, Dirk Bibcode: 2002ESOC...58..461B Altcode: 2002bcao.conf..461B No abstract at ADS 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: 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: Dynamics of the solar granulation - On the Time Variation of the Granular Flow Authors: Nesis, A.; Hammer, R.; Roth, M.; Schleicher, H.; Soltau, D.; Staiger, J. Bibcode: 2001SoPh..200...11N Altcode: The emergence and evolution of large granules shows thegranular dynamics particularly well. We therefore investigate the time dependence of the convective flows within a regular and an exploding granule. The observational material for this study was taken at the center of the solar disk with the German VTT in Izaña (Tenerife, Spain) during an observing campaign in the year 1994. It consists of series of spectrograms of high spatial resolution, which were digitized and processed with wavelet techniques. Among other features, our data show the dynamical portrait of a regular and an exploding granule. We can follow their temporal evolution over more than 12 min. Using absorption lines of different strength we are able to see the dynamical change of both granules at several heights within the first 200 km above τ5000=1. The observations reveal significant changes of the convective flow of both granules over time as well as over height, which are discussed in detail. 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: Preliminary study of the evolution of solar magnetic structures and photospheric horizontal velocity fields Authors: UeNo, S.; Kitai, R.; Ichimoto, K.; Sakurai, T.; Soltau, D.; Brandt, P. N. Bibcode: 2001AdSpR..26.1793U Altcode: In October 1997, we made a coordinated observation of the solar photosphere and chromosphere at Hida observatory (Kyoto university, Japan) and at Teide observatory (Tenerife) over a 10 day. We obtained imaging data series continuously during 6 hr 45 min in G-band (4308 Å) observed with the Domeless Solar Telescope (DST) at Hida on 24th October (effective FOV; 96″×99″). Additionally, in this observation, we simultaneously observed image series of the chromosphere during the latter 4 hr 10 min in H α line center and H α ±0.6 Å. From these data set, we could detect that emerging flux tubes crossed the photosphere to the chromosphere and that 'convective collapse' phenomena appeared at the stage of the spot formation. Moreover, we confirmed that the lifetime of mesogranulation was about 4000 sec (70 min) from the temporal evolution of velocity patterns. We show here only a summary of these observations. Title: Penumbral Stokes-V Asymmetries of Fe I 1564.8 nm Authors: Schlichenmaier, R.; Soltau, D.; Lühe, O. V. D.; Collados, M. Bibcode: 2001ASPC..236..579S Altcode: 2001aspt.conf..579S 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: Shift-and-add reconstruction of solar granulation images Authors: Baba, N.; Miura, N.; Sakurai, T.; Ichimoto, K.; Soltau, D.; Brandt, P. Bibcode: 1999SoPh..188...41B Altcode: To restore an atmospherically degraded image of solar granulation the shift-and-add (SAA) method is applied to its specklegrams. It is the first time, to the best of our knowledge, that such a technique has been used for image reconstruction of solar granulation, a largely extended target. SAA, therefore, enables us to monitor restored images of solar granulation in a simple and fast way. Title: Resolution Improvement of Solar Images Authors: Miura, Noriaki; Baba, Naoshi; Sakurai, Takashi; Ichimoto, Kiyoshi; Soltau, Dirk; Brandt, Peter Bibcode: 1999SoPh..187..347M Altcode: A method for the improvement of resolution in an observed solar image is proposed. A blind deconvolution method is used for restoration of an atmospherically-degraded solar image, and a super-resolution method is applied to its restored image to improve the resolution. It is confirmed that a blind deconvolution process can restore fine structures that are blurred in an observed image, and that the super-resolution process can make a cutoff frequency in a blind-deconvolved image higher. A time series of super-resolved images of a sunspot observed with the 70-cm Vacuum Tower Telescope at Teide Observatory is presented. Title: TIP: The Tenerife Infrared Polarimeter Authors: Martínez Pillet, V.; Collados, M.; Bellot Rubio, L. R.; Rodríiguez Hidalgo, I.; Ruiz Cobo, B.; Soltau, D. Bibcode: 1999AGAb...15...89M Altcode: 1999AGM....15..P05M The aim of the IAC Tenerife Infrared Polarimeter is to obtain quantitative measurements of the polarization state of the solar radiation collected in near-infrared wavelengths by the Gregory Coudé Telescope and the Vacuum Tower Telescope of the Observatorio del Teide (Spain). In this contribution we describe the analyzer, instrumental calibration, and detector subsystems of TIP, and give details concerning the specifications of the instrument. 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: TIP (Tenerife Infrared Polarimeter): a near IR full Stokes Polarimeter for the German Solar Telescopes at Observatorio del Teide Authors: Collados, M.; Rodríguez Hidalgo, I.; Bellot Rubio, L.; Ruiz Cobo, B.; Soltau, D. Bibcode: 1999AGAb...15Q..11C Altcode: 1999AGM....15..A13C In this contribution, the main characteristics of the=20 Tenerife Infrared Polarimeter (TIP), recently built at the Instituto de Astrofísica de Canarias, to be operated at the German Solar Telescopes (GCT and VTT) of the Observatorio del Teide (Tenerife) are presented. Especial emphasis is put on the modulation scheme, polarisation efficiencies, signal-to-noise ratio, spatial and spectral resolution, and instrumental polarisation cross-talk. Some examples of data recently obtained are presented, which give an idea of the performance and capabilities of this instrument. 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: LPSP & TIP: Full Stokes Polarimeters for the Canary Islands Observatories Authors: Mártinez Pillet, V.; Collados, M.; Sánchez Almeida, J.; González, V.; Cruz-Lopez, A.; Manescau, A.; Joven, E.; Paez, E.; Diaz, J.; Feeney, O.; Sánchez, V.; Scharmer, G.; Soltau, D. Bibcode: 1999ASPC..183..264M Altcode: 1999hrsp.conf..264M No abstract at ADS Title: The Helium Spectrum in the Quiet Sun: The January 16/17 and May 7-13 1997 Coordinated SOHO/Ground-Based Observational Campaigns Authors: Andretta, Vincenzo; Jordan, Stuart D.; Muglach, Karin; Garcia, Adriana; Jones, Harrison P.; Penn, Matthew J.; Soltau, Dirk Bibcode: 1998ASPC..155..336A Altcode: 1998sasp.conf..336A No abstract at ADS 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: Investigating the Formation of the Helium Spectrum in the Solar Atmosphere Authors: Andretta, Vincenzo; Jordan, Stuart D.; Muglach, Karin; Garcia, Adriana; Jones, Harrison P.; Soltau, Dirk Bibcode: 1998ASPC..154..559A Altcode: 1998csss...10..559A We present the first results of coordinated observations with SOHO (Solar Heliospheric Observatory) and ground-based observatories aimed at investigating the mechanisms responsible for the formation of helium lines in the quiescent solar atmosphere. The observations described here were taken on 7-13 May 1997; the SOHO instruments involved were CDS, SUMER and EIT, while ground-based support was provided by the German Vacuum Tower Telescope on Tenerife (He 1 lambda10830 and Ca 2 lambda8498 spectra-spectroheliograms), Coimbra Solar Observatory (Hα spectroheliograms), and NASA/NSO Vacuum Tower Telescope on Kitt Peak (Ca 2 lambda8542 spectra-spectroheliograms and polarimetry). Title: Stokes I and V observations of magnetic knots using bright CA II K network points as tracers. Authors: Soltau, D. Bibcode: 1997A&A...317..586S Altcode: This paper uses bright Ca II K points as markers for isolated magnetic features in the underlying solar photosphere. Stokes V polarimetry in selected photospheric lines applied to a large number of Ca II K targets yield some average results: These magnetic features are 2 to 3arcsec in diameter, dark at disk center and bright at the limb. Their magnetic field strength is typically 1400Gauss, with the field vertically oriented on average. Their magnetic filling factor is large (=~40%). Our targets share these properties with the magnetic knots introduced by Beckers & Schroeter (1968SPh.....4..142B).We therefore conclude that magnetic knots can be found anywhere on the solar disk and not only near active regions. These features may contribute significantly to the total solar magnetic flux. 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: Spectroscopic Observation of the Solar Active Regions in Hel 10830 A Line Authors: Tohmura, Ichirah; Kitai, Reizaburo; Suematsu, Yoshinori; Soltau, Dirk Bibcode: 1996JKASS..29..333T Altcode: No abstract at ADS Title: Full-field wavefront measurements with phase diversity. Authors: Acton, D. S.; Soltau, D.; Schmidt, W. Bibcode: 1996A&A...309..661A Altcode: We apply phase diversity techniques to focused and defocused images of solar granulation to make full-field measurements of the wavefront errors due to atmospheric turbulence. From these measured wavefronts, we estimate that the size of the isoplanatic patch is between 9 and 14 arcseconds. We form a two-layer model of the atmospheric turbulence in an attempt to separate the near and far-field effects. We show that this model does not adequately separate the observed turbulence, unless the measured field of view is quite large. We conclude that there is a strong layer of turbulence near the telescope, and a weaker layer of turbulence centered at about 4km above the telescope. The measured wavefronts are averaged in order to quantify the fixed aberrations present in the telescope. 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: He I 10830 Angstroms in Solar Active Regions and its Correlation with Chromospheric Lines Authors: Kitai, R.; Tohmura, I.; Suematsu, Y.; Akioka, M.; Soltau, D. Bibcode: 1996mpsa.conf..435K Altcode: 1996IAUCo.153..435K No abstract at ADS Title: Solar Observations Near 1.6 μm with the German Telescopes at Tenerife Authors: Soltau, D. Bibcode: 1995itsa.conf..407S Altcode: No abstract at ADS Title: Peculiar magnetic field evolution of active region NOAA 7562 in August 1993 - results from campaign observation with Yohkoh Authors: Sakurai, T.; Suematsu, Y.; Ichimoto, K.; Kurokawa, H.; Kitai, R.; Akioka, M.; Tohmura, I.; Soltau, D.; Mickey, D. L.; Zhang, H.; Li, W.; Zirin, H.; Tang, F. Bibcode: 1994ESASP.373..337S Altcode: 1994soho....3..337S No abstract at ADS Title: Instrumental polarization effects of the German Vacuum Tower Telescope (VTT) at Tenerife Authors: Soltau, Dirk Bibcode: 1994SPIE.2265..463S Altcode: Polarimetry is an important method to investigate the physics of the solar atmosphere. If the magnetic field strength is not strong enough to produce completely split Zeeman profiles the degree of polarization is a measure for the field strength. Measuring both the circular and linear polarization allows in principle the construction of the magnetic field vector including magnitude and direction. Unfortunately these highly desired measurements are in many cases affected by the instrument's optic itself. Especially telescopes which don't have a rotation symmetry with respect to their optical axis suffer from these problems. This is also the case with our German Vacuum Tower Telescope at Tenerife and this paper shall show the instrumental effects which are to be expected. One of the effects is crosstalk between linear and circular polarization. We show a method where this crosstalk can be considered as a tool and may be used--under certain assumptions--to derive the true size of magnetic elements which may cover only a fraction of the resolution element. Title: Sunspot observations (Invited Review) Authors: Soltau, D. Bibcode: 1994smf..conf..155S Altcode: No abstract at ADS Title: On Intensity; Magnetic Field and Velocities of Small Bright Points in the CA Network Authors: Soltau, D. Bibcode: 1993ASPC...46..225S Altcode: 1993IAUCo.141..225S; 1993mvfs.conf..225S No abstract at ADS Title: The German solar telescopes on Tenerife Authors: Soltau, Dirk Bibcode: 1991AdSpR..11e.133S Altcode: 1991AdSpR..11..133S In 1989 the last of the three German Telescopes in the Observatorio del Teide (Tenerife) the Vacuum Tower Telescope (VTT) became operational. With emphasis on this instrument a description of the telescopes is given and their performance is demonstrated. The postfocus equipment is described briefly. Title: Windows of the German Vacuum Tower Telescope. Authors: Soltau, D. Bibcode: 1991ewst.conf...23S Altcode: This paper deals with the various effects which may play a role if one introduces a plane parallel transmitting plate (i.e. a window) into an optical system. In this particular case the system is the German Vacuum Tower Telescope (VTT) which is operated on Tenerife. Only some of the possible effects are important for the VTT but this compilation together with a number of useful formulae and our experiences may help to treat this kind of problems for LEST. Title: Observations of Magnetic Features with the German Solar Telescopes at the Observatorio-Del / Tenerife Authors: Kneer, F.; Soltau, D.; Wiehr, E. Bibcode: 1990IAUS..142..113K Altcode: No abstract at ADS Title: High resolution observations with the new German VTT on Teneriffe Authors: Soltau, D. Bibcode: 1989hsrs.conf....3S Altcode: No abstract at ADS Title: The Status of the Latest German Solar Facility on Tenerife Authors: Soltau, D. Bibcode: 1989ASIC..263...17S Altcode: 1989ssg..conf...17S No abstract at ADS Title: Construction of the German Solar Telescopes in the Observatorio del Teide, Tenerife: The Vacuum Tower Telescope Authors: Schmidt, W.; Soltau, D. Bibcode: 1987MitAG..68..184S Altcode: No abstract at ADS Title: Present and Future Observational Facilities of the German Vacuum Tower Telescope Authors: Soltau, D. Bibcode: 1987rfsm.conf..362S Altcode: No abstract at ADS Title: The German solar telescopes at the observatorio del Teide Authors: Schroeter, E. H.; Soltau, D.; Wiehr, E. Bibcode: 1985VA.....28..519S Altcode: This article gives a survey of the German solar telescope installations which are going to be operational in 1985 (Gregory Coudé Telescope) and in 1986 (Vacuum Tower Telescope) respectively. Title: The German 60-cm Vacuum Tower Telescope and Its Post-Focus Facilities Authors: Soltau, D. Bibcode: 1985spit.conf.1191S Altcode: No abstract at ADS Title: Time-dependence of umbral oscillations Authors: Soltau, D.; Wiehr, E. Bibcode: 1984A&A...141..159S Altcode: Doppler shifts of the magnetically insensitive line Fe 7090.4 are observed in two sunspot umbrae on three days over time intervals up to 8.5 h. The temporal power spectra show a large variety of periods whose pattern reoccurs periodically. Neither the 300 s nor the 180 s periods are preferred umbral modes. Conspicuous power near 0.8 m Hz closely corresponds to the known 20 min time scale of umbral dots. A 240 s period from the revolution of the telescope's RA drive screw indicates the occasional existence of a structured umbral velocity field; its relation to intensity structures, e.g. the dots, is seen from the 240 s power peak in the simultaneously recorded continuum intensity. Title: Oszillationen in Flecken-Umbren Authors: Soltau, D.; Balthasar, H.; Koch, A.; Küveler, G.; Wiehr, E. Bibcode: 1983MitAG..60..225S Altcode: No abstract at ADS Title: A morphological study of some umbral fine structures Authors: Soltau, D. Bibcode: 1982A&A...107..211S Altcode: Photographic analysis of a high resoluted white-light photograph of a big sunspot (Mount Wilson No. 20551) shows that there is a morphological relation between bright penumbral filaments and umbral dots. Within the umbra evidence is found for the existence of features which are considerably bigger than umbral dots and which may be identified with earlier found 'umbral granulation'. Title: Guiding and Image Stabilization of the 60 cm VTT by Means of a Fourth-Order Servo System Authors: Soltau, D. Bibcode: 1981siwn.conf..600S Altcode: No abstract at ADS Title: An attempt to compare the differential rotation of the Ca+-network with that of the photospheric plasma. Authors: Schroeter, E. H.; Woehl, H.; Soltau, D.; Vazquez, M. Bibcode: 1978SoPh...60..181S Altcode: In this third paper of a series we report on results obtained from almost simultaneous observations of the differential rotation of the Ca+-chromosphere and the photospheric plasma. The observations (tracings of Ca+-mottles, Doppler-shift-measurements in a photospheric line) were performed at the Locarno station during two extended periods in 1976. Both data sets were used to search for a large scale circulation pattern. A regular long-lived pattern could not be detected, but a single cell with radial outflow (∼40 ms-1) extending over more than 50° in longitude has been found. Statistically significant temporal changes of the differential rotation law of the Ca+-chromosphere correlated with changes of meridional motions and differences in the rotation of the two hemispheres were observed. The data of 1976 show no significant difference in the equatorial velocity of the Ca+-chromosphere and the photosphere. A comparison of temporal variations of the rotation velocity of the Ca+-mottles and the photospheric plasma yielded no significant correlation. Finally, we correlated the measurements of the photospheric rotation performed within the equatorial belts at Locarno with those obtained at Mt. Wilson by a different method during the same periods. The Locarno data show small temporal variations (±50 m-1s) with no correlation between the two hemispheres, while from the Mt. Wilson data larger variations (±100 m s-1) with a very high correlation between the two hemispheres have been found. A comparison of both sets of data yielded no correlation at all demonstrating the need for further coordinated observations and a drastic increase of internal accuracy. Title: Zum Problem der Evershedströmungen in Penumbra-Feinstrukturen Authors: Wiehr, E.; Stellmacher, G.; Soltau, D. Bibcode: 1977MitAG..42..117W Altcode: The use of a two-flow model in the interpretation of the solar Evershed flows reported by Schroeter (1965) leads to contradictions with respect to recent measurements regarding the flow characteristics in the penumbra fine structures. Stellmacher and Wiehr (1971) have found in an analysis of line asymmetries based on such a model that the line main component must be related to slow-flowing bright penumbra structures. The line satellite, on the other hand, must be related to fast-flowing dark penumbra structures. A series of nonsplitting lines in two long-term individual spots were recorded in connection with a more detailed investigation of this model concept. The asymmetrical penumbral line profiles were decomposed with the aid of a computer calculation into a main component and a satellite. The significance of the results is discussed. It is found that the observational phenomena can be very well reconciled with a depth-dependent flow concept for the penumbra proposed by Maltby (1964). Title: On velocity oscillations in sunspot umbrae. Authors: Soltau, D.; Schroeter, E. H.; Woehl, H. Bibcode: 1976A&A....50..367S Altcode: We report results from a power spectrum analysis of photographic measurements of velocity oscillations in two sunspot umbrae. We find oscillations with 3 regimes, of periods: 470-300 s, 196-164 s and 123-110 s with a peak to peak amplitude of about 0.6 km/sec. Whereas the two long period oscillations agree well with the findings of previous authors the detection of the short period oscillations is new. We find significant power for the 300 s oscillations in a purely umbral molecular line, 6496.2 A (TiO), which establishes the umbral origin of this oscillation mode. Title: On the time behaviour of oscillations in sunspot umbrae. Authors: Schroeter, E. H.; Soltau, D. Bibcode: 1976A&A....49..463S Altcode: The time behavior of oscillations in a sunspot umbra is studied using Doppler-compensator velocity records with high time resolution. A power-spectrum analysis yields modes with different periods. It is found that during time periods when the spot umbra oscillated with the 180-s period, the 300-s period was strongly suppressed and vice versa.