Author name code: suetterlin ADS astronomy entries on 2022-09-14 author:"Suetterlin, Peter" ------------------------------------------------------------------------ Title: Design and Performance Analysis of a Highly Efficient Polychromatic Full Stokes Polarization Modulator for the CRISP Imaging Spectrometer Authors: de Wijn, A. G.; de la Cruz Rodríguez, J.; Scharmer, G. B.; Sliepen, G.; Sütterlin, P. Bibcode: 2021AJ....161...89D Altcode: 2021arXiv210201231D We present the design and performance of a polychromatic polarization modulator for the CRisp Imaging SpectroPolarimeter (CRISP) Fabry-Perot tunable narrow-band imaging spectropolarimer at the Swedish 1 m Solar Telescope (SST). We discuss the design process in depth, compare two possible modulator designs through a tolerance analysis, and investigate thermal sensitivity of the selected design. The trade-offs and procedures described in this paper are generally applicable in the development of broadband polarization modulators. The modulator was built and has been operational since 2015. Its measured performance is close to optimal between 500 and 900 nm, and differences between the design and as-built modulator are largely understood. We show some example data, and briefly review scientific work that used data from SST/CRISP and this modulator. Title: CRISPRED: CRISP imaging spectropolarimeter data reduction pipeline Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.; Hillberg, T.; Rouppe van der Voort, L. Bibcode: 2017ascl.soft08003D Altcode: CRISPRED reduces data from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope (SST). It performs fitting routines, corrects optical aberrations from atmospheric turbulence as well as from the optics, and compensates for inter-camera misalignments, field-dependent and time-varying instrumental polarization, and spatial variation in the detector gain and in the zero level offset (bias). It has an object-oriented IDL structure with computationally demanding routines performed in C subprograms called as dynamically loadable modules (DLMs). Title: CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.; Hillberg, T.; Rouppe van der Voort, L. Bibcode: 2015A&A...573A..40D Altcode: 2014arXiv1406.0202D The production of science-ready data from major solar telescopes requires expertise beyond that of the typical observer. This is a consequence of the increasing complexity of instruments and observing sequences, which require calibrations and corrections for instrumental and seeing effects that are not only difficult to measure, but are also coupled in ways that require careful analysis in the design of the correction procedures. Modern space-based telescopes have data-processing pipelines capable of routinely producing well-characterized data products. High resolution imaging spectropolarimeters at ground-based telescopes need similar data pipelines.We present new methods for flat-fielding spectropolarimetric data acquired with telecentric Fabry-Perot instruments and a new approach for accurate camera co-alignment for image restoration. We document a procedure that forms the basis of current state-of-the-art processing of data from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope (SST). By collecting, implementing, and testing a suite of computer programs, we have defined a data reduction pipeline for this instrument. This pipeline, CRISPRED, streamlines the process of making science-ready data.It is implemented and operated in IDL, with time-consuming steps delegated to C.CRISPRED will also be the basis for the data pipeline of the forthcoming CHROMIS instrument. Title: Ellerman bombs: fallacies, fads, usage Authors: Rutten, Robert J.; Vissers, Gregal J. M.; Rouppe van der Voort, Luc H. M.; Sütterlin, Peter; Vitas, Nikola Bibcode: 2013JPhCS.440a2007R Altcode: 2013arXiv1304.1364R Ellerman bombs are short-lived brightenings of the outer wings of Hα that occur in active regions with much flux emergence. We point out fads and fallacies in the extensive Ellerman bomb literature, discuss their appearance in various spectral diagnostics, and advocate their use as indicators of field reconfiguration in active-region topography using AIA 1700 Å images. Title: Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra Authors: Scharmer, G. B.; de la Cruz Rodriguez, J.; Sütterlin, P.; Henriques, V. M. J. Bibcode: 2013A&A...553A..63S Altcode: 2012arXiv1211.5776S We discuss NICOLE inversions of Fe i 630.15 nm and 630.25 nm Stokes spectra from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial resolution close to 0.15 arcsec. We report on narrow, radially extended lanes of opposite polarity field, located at the boundaries between areas of relatively horizontal magnetic field (the intra-spines) and much more vertical field (the spines). These lanes harbor convective downflows of about 1 km s-1. The locations of these downflows close to the spines agree with predictions from the convective gap model (the "gappy penumbra") proposed six years ago, and more recent three-dimensional magnetohydrodynamic simulations. We also confirm the existence of strong convective flows throughout the entire penumbra, showing the expected correlation between temperature and vertical velocity, and having vertical root mean square velocities of about 1.2 km s-1. Title: Search for Alfvén waves in a bright network element observed in Hα Authors: Koza, J.; Sütterlin, P.; Gömöry, P.; Rybák, J.; Kučera, A. Bibcode: 2013CoSka..43....5K Altcode: 2013arXiv1304.4027K Alfvén waves are considered as potential transporters of energy heating the solar corona. We seek spectroscopic signatures of the Alfvén waves in the chromosphere occupied by a bright network element, investigating temporal variations of the spectral width, intensity, Dopplershift, and the asymmetry of the core of the Hα spectral line observed by the tunable Lyot filter installed on the Dutch Open Telescope. The spectral characteristics are derived through the fitting of five intensity samples, separated from each other by 0.35 Å, by a 4th-order polynomial. The bright network element displays the most pronounced variations of the Dopplershift varying from 0 to 4 km s-1 about the average of 1.5 km s-1. This fact implies a persistent redshift of the Hα core with a redward asymmetry of about 0.5 km s-1, suggesting an inverse-C bisector. The variations of the core intensity up to ±10 % and the core width up to ±5 % about the respective averages are much less pronounced, but still detectable. The core intensity variations lag behind the Dopplershift variations about 2.1 min. The Hα core width tends to correlate with the Dopplershift and anticorrelate with the asymmetry, suggesting that more redshifted Hα profiles are wider and the broadening of the Hα core is accompanied with a change of the core asymmetry from redward to blueward. We also found a striking anticorrelation between the core asymmetry and the Dopplershift, suggesting a change of the core asymmetry from redward to blueward with an increasing redshift of the Hα core. The data and the applied analysis do not show meaningful tracks of Alfvén waves in the selected network element. Title: Large-field high-resolution mosaic movies Authors: Hammerschlag, Robert H.; Sliepen, Guus; Bettonvil, Felix C. M.; Jägers, Aswin P. L.; Sütterlin, Peter; Martin, Sara F. Bibcode: 2012SPIE.8444E..06H Altcode: Movies with fields-of-view larger than normal for high-resolution telescopes will give a better understanding of processes on the Sun, such as filament and active region developments and their possible interactions. New active regions can influence, by their emergence, their environment to the extent of possibly serving as an igniter of the eruption of a nearby filament. A method to create a large field-of-view is to join several fields-of-view into a mosaic. Fields are imaged quickly one after another using fast telescope-pointing. Such a pointing cycle has been automated at the Dutch Open Telescope (DOT), a high-resolution solar telescope located on the Canary Island La Palma. The observer can draw with the computer mouse the desired total field in the guider-telescope image of the whole Sun. The guider telescope is equipped with an H-alpha filter and electronic enhancement of contrast in the image for good visibility of filaments and prominences. The number and positions of the subfields are calculated automatically and represented by an array of bright points indicating the subfield centers inside the drawn rectangle of the total field on the computer screen with the whole-sun image. When the exposures start the telescope repeats automatically the sequence of subfields. Automatic production of flats is also programmed including defocusing and fast motion over the solar disk of the image field. For the first time mosaic movies were programmed from stored information on automated telescope motions from one field to the next. The mosaic movies fill the gap between whole-sun images with limited resolution of synoptic telescopes including space instruments and small-field high-cadence movies of high-resolution solar telescopes. Title: Sunlight refraction in the mesosphere of Venus during the transit on June 8th, 2004 Authors: Tanga, P.; Widemann, T.; Sicardy, B.; Pasachoff, J. M.; Arnaud, J.; Comolli, L.; Rondi, A.; Rondi, S.; Sütterlin, P. Bibcode: 2012Icar..218..207T Altcode: 2011arXiv1112.3136T Many observers in the past gave detailed descriptions of the telescopic aspect of Venus during its extremely rare transits across the Solar disk. In particular, at the ingress and egress, the portion of the planet’s disk outside the Solar photosphere has been repeatedly perceived as outlined by a thin, bright arc (“aureole”). Those historical visual observations allowed inferring the existence of Venus’ atmosphere, the bright arc being correctly ascribed to the refraction of light by the outer layers of a dense atmosphere. On June 8th, 2004, fast photometry based on electronic imaging devices allowed the first quantitative analysis of the phenomenon. Several observers used a variety of acquisition systems to image the event - ranging from amateur-sized to professional telescopes and cameras - thus collecting for the first time a large amount of quantitative information on this atmospheric phenomenon. In this paper, after reviewing some elements brought by the historical records, we give a detailed report of the ground based observations of the 2004 transit. Besides confirming the historical descriptions, we perform the first photometric analysis of the aureole using various acquisition systems. The spatially resolved data provide measurements of the aureole flux as a function of the planetocentric latitude along the limb. A new differential refraction model of solar disk through the upper atmosphere allows us to relate the variable photometry to the latitudinal dependency of scale-height with temperature in the South polar region, as well as the latitudinal variation of the cloud-top layer altitude. We compare our measurements to recent analysis of the Venus Express VIRTIS-M, VMC and SPICAV/SOIR thermal field and aerosol distribution. Our results can be used a starting point for new, more optimized experiments during the 2012 transit event. Title: Multi-wavelength fine structure and mass flows in solar microflares Authors: Berkebile-Stoiser, S.; Gömöry, P.; Veronig, A. M.; Rybák, J.; Sütterlin, P. Bibcode: 2009A&A...505..811B Altcode: Aims: We study the multi-wavelength characteristics at high spatial resolution, as well as chromospheric evaporation signatures of solar microflares. To this end, we analyze the fine structure and mass flow dynamics in the chromosphere, transition region and corona of three homologous microflares (GOES class <A9/0.7 with/without background), which occurred on July 4, 2006 in AR 10898.
Methods: A multi-wavelength analysis using temporally and spatially highly resolved imaging data from the Dutch open telescope (Hα, Ca ii H), the transition region and coronal explorer (17.1 nm), the extreme-ultraviolet imaging telescope (19.5 nm), and the Reuven Ramaty high energy solar spectroscopic imager (≳3 keV) was carried out. EUV line spectra provided by the coronal diagnostic spectrometer are searched for Doppler shifts in order to study associated plasma flows at chromospheric (He i, T∼3.9× 104 K), transition region (e.g. O v, T∼ 2.6× 105 K), and coronal temperatures (Si xii, T∼ 2× 106 K). RHESSI X-ray spectra provide information about non-thermal electrons.
Results: The multi-wavelength appearance of the microflares is in basic agreement with the characteristics of large flares. For the first event, a complex flare sequence is observed in TRACE 17.1 nm images (T≈ 1 MK), which show several brightenings, narrow loops of enhanced emission, and an EUV jet. EIT 19.5 nm data (T≈ 1.5 MK) exhibit similar features for the third event. DOT measurements show finely structured chromospheric flare brightenings for all three events, loop-shaped fibrils of increased emission between Hα brightenings, as well as a similar feature in Ca ii. For all three events, a RHESSI X-ray source (3-8 keV, T ≳ 10 MK) is located in between two chromospheric brightenings situated in magnetic flux of opposite polarity. We find the flow dynamics associated with the events to be very complex. In the chromosphere and transition region, CDS observed downflows for the first (v ≲ 40 km s-1), and upflows for the second event (v ≲ 40 km s-1). During the third microflare, we find upflows of ≲ 20 km s-1 and also weak downflows of ≲20 km s-1 in two separate brightenings. For all three microflares, multi-component fitting is needed for several profiles of He i, O v, and Ne vi lines observed at the flare peaks, which indicate spatially unresolved, oppositely directed flows of ≲180 km s-1. We interpret these flows as twisting motions of the flare loops. Loop-shaped fibrils in between Hα brightenings showing opposite flow directions (v≈5 km s-1) are also observed in DOT Hα Dopplergrams. RHESSI X-ray spectra show evidence of non-thermal bremsstrahlung for two of the three microflares. The electron beam flux density deposited in the chromosphere for these events is estimated to straddle the threshold heating flux between gentle and explosive evaporation.

Appendix A and the movie are only available in electronic form at http://www.aanda.org Title: Multi-wavelength Observations of Microflares Near an Active Region Authors: Bein, B.; Veronig, A.; Rybak, J.; Gömöry, P.; Berkebile-Stoiser, S.; Sütterlin, P. Bibcode: 2009CEAB...33..179B Altcode: We study the multi-wavelength characteristics of a microflaring active region (AR 10898) near disc centre. The analysed data were from the 4^{th} of July 2006, and were recorded by DOT (Hα, Ca II H), RHESSI (X-rays), TRACE (EUV) and SOHO/MDI (magnetograms). The identified microflare events were studied with respect to their magnetic field configuration and their multi-wavelength time evolution. Title: CRISP Spectropolarimetric Imaging of Penumbral Fine Structure Authors: Scharmer, G. B.; Narayan, G.; Hillberg, T.; de la Cruz Rodriguez, J.; Löfdahl, M. G.; Kiselman, D.; Sütterlin, P.; van Noort, M.; Lagg, A. Bibcode: 2008ApJ...689L..69S Altcode: 2008arXiv0806.1638S We discuss penumbral fine structure in a small part of a pore, observed with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST), close to its diffraction limit of 0.16''. Milne-Eddington inversions applied to these Stokes data reveal large variations of field strength and inclination angle over dark-cored penumbral intrusions and a dark-cored light bridge. The mid-outer part of this penumbra structure shows ~0.3'' wide spines, separated by ~1.6'' (1200 km) and associated with 30° inclination variations. Between these spines, there are no small-scale magnetic structures that easily can be identified with individual flux tubes. A structure with nearly 10° more vertical and weaker magnetic field is seen midway between two spines. This structure is cospatial with the brightest penumbral filament, possibly indicating the location of a convective upflow from below. Title: Photospheric flows around a quiescent filament at Large and small scale and their ffects on filament destabilization Authors: Roudier, Th.; Malherbe, J. M.; Švanda, M.; Molodij, G.; Keil, S.; Sütterlin, P.; Schmieder, B.; Bommier, V.; Aulanier, G.; Meunier, N.; Rieutord, M.; Rondi, S. Bibcode: 2008sf2a.conf..569R Altcode: We study the influence of large and small scales photospheric motions on the destabilization of an eruptive filament, observed on October 6, 7, and 8, 2004 as part of an international observing campaign (JOP 178). Large-scale horizontal flows are invetigated from a series of MDI/SOHO full-disc Dopplergrams and magnetograms from THEMIS. Small-scale horizontal flows were derived using local correlation tracking on TRACE satellite, Dutch Open Telescope (DOT) and The Dunn Solar telescope (DST) data. The topology of the flow field changed significantly during the filament eruptive phase, suggesting a possible coupling between the surface flow field and the coronal magnetic field. We measured an increase of the shear below the point where the eruption starts and a decrease in shear after the eruption. We conclude that there is probably a link between changes in surface flow and the disappearance of the eruptive filament. Title: Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution Authors: Scharmer, G.; Henriques, V.; Hillberg, T.; Kiselman, D.; Löfdahl, M.; Narayan, G.; Sütterlin, P.; van Noort, M.; de la Cruz Rodríguez, J. Bibcode: 2008ESPM...12..2.5S Altcode: We present first observations of sunspots with the imaging spectropolarimeter CRISP, recently installed at the Swedish 1-m Solar Telescope (SST) on La Palma. This spectropolarimeter is based on a high-fidelity dual Fabry-Perot filter system.

Two liquid crystals and a polarizing beam splitter are used to reduce seeing induced I,Q,U,V crosstalk by simultaneously recording images with two 1kx1k back-illuminated Sarnoff CCD's. A third CCD simultaneously records broadband images through the pre-filter of the FPI filter system, allowing image reconstruction and co-alignment of images of different polarization states and at different wavelengths in Zeeman sensitive spectral lines.

The first data, recorded in April 2008, demonstrate the capability of this system to record high cadence, high S/N polarimetric data with a spatial resolution at or close to the diffraction limit of the SST at 630 nm, 0.16 arcsec. We discuss the analysis of first spectropolarimetric data for sunspots, based on Milne-Eddington inversion techniques. Title: DOT Tomography of the Solar Atmosphere VII. Chromospheric Response to Acoustic Events Authors: Rutten, R. J.; van Veelen, B.; Sütterlin, P. Bibcode: 2008SoPh..251..533R Altcode: 2008arXiv0801.0374R; 2008SoPh..tmp...28R We use synchronous movies from the Dutch Open Telescope sampling the G band, Ca II H, and Hα with five-wavelength profile sampling to study the response of the chromosphere to acoustic events in the underlying photosphere. We first compare the visibility of the chromosphere in Ca II H and Hα, demonstrate that studying the chromosphere requires Hα data, and summarize recent developments in understanding why this is so. We construct divergence and vorticity maps of the photospheric flow field from the G-band images and locate specific events through the appearance of bright Ca II H grains. The reaction of the Hα chromosphere is diagnosed in terms of brightness and Doppler shift. We show and discuss three particular cases in detail: a regular acoustic grain marking shock excitation by granular dynamics, a persistent flasher, which probably marks magnetic-field concentration, and an exploding granule. All three appear to buffet overlying fibrils, most clearly in Dopplergrams. Although our diagnostic displays to dissect these phenomena are unprecedentedly comprehensive, adding even more information (photospheric Doppler tomography and magnetograms along with chromospheric imaging and Doppler mapping in the ultraviolet) is warranted. Title: Dynamic Fibrils in Ly-alpha Authors: Koza, J.; Rutten, R. J.; Vourlidas, A.; Suetterlin, P. Bibcode: 2008ESPM...12.2.16K Altcode: We have detected dynamic fibrils (DFs) in Ly-alpha filtergrams taken with the rocket-borne Very high Angular resolution ULtraviolet Telescope (VAULT). Although the data consist of only a 1-min sequence of 4 images taken near the solar limb during the second VAULT flight, they enable us to identify and study the time evolution of over 50 DFs. Most show parabolic trajectories in their angular extent, with supersonic maximum velocities. The measured decelerations vary from sub-ballistic to super-ballistic. The similarities with DFs seen in Halpha suggest a common cause, possibly the presence of hot transition-region interfaces around cool oscillation-fed jets. Title: SST/CRISP Magnetometry with Fe I 630.2 nm Authors: Narayan, G.; Scharmer, G. B.; Hillberg, T.; Lofdahl, M.; van Noort, M.; Sutterlin, P.; Lagg, A. Bibcode: 2008ESPM...122.120N Altcode: We present recent full Stokes observations in the Fe I 630.2 nm line with CRISP, an imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST). The observations reach a spatial resolution of 0".16, close to the diffraction limit of the SST, representing a major improvement over any past ground based or space based spectropolarimetric data. We describe the data acquisition and reduction methods and present results of Milne-Eddington(ME) inversions applied on observations of plage. Title: Search for photospheric footpoints of quiet Sun transition region loops Authors: Sánchez Almeida, J.; Teriaca, L.; Sütterlin, P.; Spadaro, D.; Schühle, U.; Rutten, R. J. Bibcode: 2007A&A...475.1101S Altcode: 2007arXiv0709.3451S Context: The footpoints of quiet Sun Transition Region (TR) loops do not seem to coincide with the photospheric magnetic structures appearing in traditional low-sensitivity magnetograms.
Aims: We look for the so-far unidentified photospheric footpoints of TR loops using G-band bright points (BPs) as proxies for photospheric magnetic field concentrations.
Methods: We compare TR measurements with SoHO/SUMER and photospheric magnetic field observations obtained with the Dutch Open Telescope.
Results: Photospheric BPs are associated with bright TR structures, but they seem to avoid the brightest parts of the structure. BPs appear in regions that are globally redshifted, but they avoid extreme velocities. TR explosive events are not clearly associated with BPs.
Conclusions: The observations are not inconsistent with the BPs being footpoints of TR loops, although we have not succeeded to uniquely identify particular BPs with specific TR loops. Title: Magnetic properties of G-band bright points in a sunspot moat Authors: Beck, C.; Bellot Rubio, L. R.; Schlichenmaier, R.; Sütterlin, P. Bibcode: 2007A&A...472..607B Altcode: 2007arXiv0707.1232B We present simultaneous spectropolarimetric observations of four visible (630 nm) and three infrared (1565 nm) spectral lines from the German Vacuum Tower Telescope, together with speckle-reconstructed filtergrams in the G-band and the Ca II H line core from the Dutch Open Telescope. After alignment of the data sets, we used the G-band intensity to locate bright points (BPs) in the moat of a regular sunspot. With the cospatial and cotemporal information provided by the polarimetric data, we characterize the magnetic, kinematic, and thermal properties of the BPs. We find that (a) 94% of the BPs are associated with magnetic fields; (b) their field strengths range between 500 and 1400 G, with a rather flat distribution; (c) the contrast of BPs in the G-band depends on the angle between the vector magnetic field and the line of sight; (d) the BPs harbor downflows of magnetized plasma and exhibit Stokes V profiles with large area and amplitude asymmetries; (e) the magnetic interior of BPs is hotter than the immediate field-free surroundings by about 1000 K at equal optical depth; and (f) the mean effective diameter of BPs in our data set is 150 km, with very few BPs larger than 300 km. Most of these properties can be explained by the classical magnetic flux tube model. However, the wide range of BP parameters found in this study indicates that not all G-band BPs are identical to stable long-lived flux tubes or sheets of kG strength.

Appendices A-C are only available in electronic form at http://www.aanda.org Title: Photospheric flows around a quiescent filament Authors: Rondi, S.; Roudier, Th.; Molodij, G.; Bommier, V.; Keil, S.; Sütterlin, P.; Malherbe, J. M.; Meunier, N.; Schmieder, B.; Maloney, P. Bibcode: 2007A&A...467.1289R Altcode: Context: The horizontal photospheric flows below and around a filament are one of the components in the formation and evolution of filaments. Few studies exist because they require multiwalength time sequences at high spatial resolution.
Aims: Our objective is to measure the horizontal photospheric flows associated with the evolution and eruption of a filament.
Methods: We present observations obtained in 2004 during the international JOP 178 campaign which involved eleven instruments both in space and at ground based observatories. We use TRACE WL, DOT and DST observation to derive flow maps which are then coaligned with intensity images and with the vector magnetic field map obtained with THEMIS/MTR.
Results: Several supergranulation cells cross the Polarity Inversion Line (PIL) and can transport magnetic flux through the PIL, in particular parasitic polarities. We present a detailed example of the formation of a secondary magnetic dip at the location of a filament footpoint. Large-scale converging flows, which could exist along the filament channel and contribute to its formation, are not observed. Before the filament's eruptive phase, we observe both parasitic and normal polarities being swept by a continuously diverging horizontal flow located in the filament gap. The disappearance of the filament initiates in this gap. Such purely horizontal motions could lead to destabilization of the filament and could trigger the sudden filament disappearance. Title: Temporal Variations in Fibril Orientation Authors: Koza, J.; Sütterlin, P.; Kučera, A.; Rybák, J. Bibcode: 2007ASPC..368..115K Altcode: 2007astro.ph..3733K We measure variations in orientation of fourteen dynamic fibrils as a function of time in a small isolated plage and nearby network using a 10-min time sequence of Hα filtergrams obtained by the Dutch Open Telescope. We found motions with average angular velocities of the order of 1 deg min-1 suggesting systematic turning from one limit position to another, particularly apparent in the case of fibrils with lifetimes of a few minutes. Shorter fibrils tend to turn faster than longer ones, which we interpret as due to vortex flows in the underlying granulation that twist magnetic fields. Title: Magnetic properties of G-band bright points Authors: Beck, C.; Mikurda, K.; Bellot Rubio, L. R.; Schlichenmaier, R.; Sütterlin, P. Bibcode: 2007msfa.conf..165B Altcode: Bright points (BPs) visible in the G band at 430 nm are commonly used as tracers of magnetic fields, indicating the location of kG flux concentrations. To study the actual magnetic properties of G-band BPs, we took observations in 2003 and 2005, employing simultaneously a speckle setup in the G band and vector spectropolarimetry to derive the magnetic field vector. From the analysis of the co-aligned polarimetric data we find that the BPs show a broad range of field strengths, magnetic fluxes, and field inclinations. Many G-band BPs are not co-spatial with the central part of the nearby flux concentrations. Even at the small heliocentric angle of only 12°, the BPs appear projected on adjacent granules, whereas the magnetic field is concentrated in the intergranular lanes. Our findings support the view that the G-band BPs are a result of the "hot wall effect". The downward shift of the optical depth scale in the presence of magnetic fields allows to see deeper and hotter layers in the hot granules next to the field concentrations, where CH dissociates. Thus, information drawn from imaging observations of BPs has limited use to investigate the actual magnetic field structure, when the BPs are not co-spatial with the central part of the flux concentrations. Title: Polarimetric Observations of the Formation of a G-Band Bright Point Authors: Beck, C.; Schmidt, W.; Bellot Rubio, L. R.; Schlichenmaier, R.; Sütterlin, P.; Lites, B. W. Bibcode: 2006ASPC..358...72B Altcode: We investigate the kinematic and magnetic properties of G-band bright points in the moat of a regular sunspot. The analysis is based on vector polarimetric measurements made at the German Vacuum Tower Telescope in visible (630 nm) and infrared (1565 nm) spectral lines, complemented by high-resolution filtergrams in the G-band at 430.6 nm and the core of the Ca II H line at 396.7 nm from the Dutch Open Telescope. The spectro-polarimetric data has been inverted to derive the magnetic field properties of the observed region. We witness the formation of a G-band bright point from a patch of diffuse flux with an initial field strength of 0.4 kG. The magnetic field strength increases to 0.9 kG in the course of several minutes, accompanied by a downflow of magnetized plasma. A few minutes after the field intensification, a G-band bright point is seen at the location of the flux concentration. The formation of the bright point shows the signatures of convective collapse. Title: Magnetic Patches in Internetwork Areas Authors: de Wijn, A. G.; Rutten, R. J.; Haverkamp, E. M. W. P.; Sütterlin, P. Bibcode: 2006ASPC..354...20D Altcode: We present a study of internetwork magnetic elements that appear as bright points in G-band (photosphere) and Ca II H (low chromosphere) image sequences from the Dutch Open Telescope. Many bright points appear intermittently in groups of long-lived structures that we call ``magnetic patches''. We develop an algorithm for the identification of bright points and magnetic patches. The average internetwork bright point lifetimes is measured to be 3.5 minutes in the G band, and 4.3 minutes in the Ca II H. We find an internetwork bright point number density of 0.02 Mm^{-2} in the G-band sequence and 0.05 Mm^{-2} in the Ca II H sequence. The bright points show a bimodal distribution of the frame-to-frame horizontal velocities, with a peak at 0 km s^{-1} and a wide hump centered around 1.2 km s^{-1}. The patches last much longer than granular time scales (about nine hours) and outline cell-like structures on mesogranular scale. We conclude that transient internetwork bright points trace the locations of strong magnetic fields that exist before the bright point appears and remain after it disappears. Title: Tunable H-alpha Lyot filter with advanced servo system and image processing: instrument design and new scientific results with the Dutch Open Telescope Authors: Bettonvil, Felix C. M.; Hammerschlag, Robert H.; Sütterlin, Peter; Rutten, Robert J.; Jägers, Aswin P. L.; Sliepen, Guus Bibcode: 2006SPIE.6269E..0EB Altcode: 2006SPIE.6269E..12B The Dutch Open Telescope (DOT; http://dot.astro.uu.nl) on La Palma is a revolutionary open solar telescope, on an excellent site, on top of a transparent tower of steel framework, and uses natural air flow to minimize local seeing. The DOT is a high-resolution multi-wavelength imager capable of long-duration time series aiming at magnetic fine structure, topology and dynamics in the photosphere and low- and high chromosphere. In this paper we describe the latest addition to the multi-wavelength imaging system: a Lyot H-alpha camera channel operating at a wavelength of 656.3 nm, being of major interest for high-chromospheric phenomena. The channel is operated strictly synchronous with the other channels and all data are speckle reconstructed. The channel permits profile sampling and delivers Dopplergrams in a 15 second time cadence, up to several hours long and adding up to a total data amount of 1.6 Terabyte/day. A dedicated computer (DSP, DOT Speckle Processor) has been built for processing the data overnight. Title: DOT tomography of the solar atmosphere. VI. Magnetic elements as bright points in the blue wing of Hα Authors: Leenaarts, J.; Rutten, R. J.; Sütterlin, P.; Carlsson, M.; Uitenbroek, H. Bibcode: 2006A&A...449.1209L Altcode: High-resolution solar images taken in the blue wing of the Balmer H α line with the Dutch Open Telescope show intergranular magnetic elements as strikingly bright features, similar to, but with appreciably larger contrast over the surrounding granulation than their more familiar manifestation as G-band bright points. Part of this prominent appearance is due to low granular contrast, without granule/lane brightness reversal as, e.g., in the wings of Ca II H & K. We use 1D and 2D radiative transfer modeling and 3D solar convection and magnetoconvection simulations to reproduce and explain the H α wing images. We find that the blue H α wing obeys near-LTE line formation. It appears particularly bright in magnetic elements through low temperature gradients. The granulation observed in the blue wing of H α has low contrast because of the lack of H α opacity in the upper photosphere, Doppler cancellation, and large opacity sensitivity to temperature working against source function sensitivity. We conclude that the blue H α wing represents a promising proxy magnetometer to locate and track isolated intermittent magnetic elements, a better one than the G band and the wings of Ca II H & K although less sharp at given aperture. Title: Unveiling the atmosphere of Venus during the June 2004 solar transit Authors: Tanga, P.; Arnaud, J.; Colas, F.; Comolli, L.; Rondi, S., A.; Sicardy, B.; Suetterlin, P.; Unione Astrofili Italiani-``Planets" Section Team Bibcode: 2005DPS....37.5718T Altcode: 2005BAAS...37.1571T Visual and CCD observations of Venus transiting in front of the Sun on June 8, 2004, revealed traces of the light passing through the atmosphere of the planet. As several witness of past events have accounted for [1][2], the event happens close to the ingress and egress phases, when the disk of the planet is crossing the Solar limb. The portion of atmosphere that is projected against the darker sky background appears bright, at least in part, due to refracted light (mainly).

The 2004 opportunity was the first observed by electronic equipment. Sets of several images were acquired by widely different instruments, at different wavelengths.

The images were processed in order to subtract the sky background, and normalized. The brightness of the atmospheric arc was measured; its spatial structure and its variation in time are discussed. A latitude dependence of the arc brightness is clearly detected, with an intensity maximum close to the planet polar regions. Even comparing CCD images to visual observations, an investigation of the variability of the arc brightness from one event to the other encounters several difficulties, mainly due to the evolution of instruments in time.

[1] F. Link, Eclipse phenomena in Astronomy, Springer Verlag (Berlin 1969)

[2] H.N. Russell, ApJ 9, 284 (1899) Title: DOT tomography of the solar atmosphere. V. Analysis of a surge from AR10486 Authors: Tziotziou, K.; Tsiropoula, G.; Sütterlin, P. Bibcode: 2005A&A...444..265T Altcode: We present an analysis of high temporal and spatial resolution CaII H chromospheric limb observations obtained with the Dutch Open Telescope (DOT). We focus on a solar surge observed both by the DOT in CaII H and the Transition Region and Coronal Explorer (TRACE) satellite in the 195 Å and 1600 Å passbands. The surge is observed in active region AR10486 located near the solar limb, a region which two hours later produced the largest X-flare ever recorded. It consists of relatively cold gas of about 104-105 K. In TRACE images the surge is followed for almost 2.5 h, shrinking and expanding at the same location several times. From DOT images we find outward propagating intensity disturbances, with velocities higher than 50 km s-1, indicative of upward material motion. The latter is also suggested by the good correlation between the DOT and TRACE surge apparent height curves, their apparent time delay and a phase difference analysis. A spectral wavelet analysis of the brightness variations within and along the surge shows a predominant period of ~6 min, the first ever reported for this kind of structures. Magnetic reconnection at the bottom of the surge as its driving mechanism is suggested by the observed inverted "Y" shape configuration and is further supported by a phase difference analysis. Title: Small Scale Magnetic Elements as Bright Points in the Blue Hα Wing Authors: Leenaarts, J.; Sütterlin, P.; Rutten, R. J.; Carlsson, M.; Uitenbroek, H. Bibcode: 2005ESASP.596E..15L Altcode: 2005ccmf.confE..15L No abstract at ADS Title: DOT tomography of the solar atmosphere. IV. Magnetic patches in internetwork areas Authors: de Wijn, A. G.; Rutten, R. J.; Haverkamp, E. M. W. P.; Sütterlin, P. Bibcode: 2005A&A...441.1183D Altcode: 2007arXiv0706.2008D We use G-band and Ca ii H image sequences from the Dutch Open Telescope (DOT) to study magnetic elements that appear as bright points in internetwork parts of the quiet solar photosphere and chromosphere. We find that many of these bright points appear recurrently with varying intensity and horizontal motion within longer-lived magnetic patches. We develop an algorithm for detection of the patches and find that all patches identified last much longer than the granulation. The patches outline cell patterns on mesogranular scales, indicating that magnetic flux tubes are advected by granular flows to mesogranular boundaries. Statistical analysis of the emergence and disappearance of the patches points to an average patch lifetime as long as 530±50~min (about nine hours), which suggests that the magnetic elements constituting strong internetwork fields are not generated by a local turbulent dynamo. Title: DOT++: the Dutch Open Telescope with 1.4-m aperture Authors: Bettonvil, Felix C.; Hammerschlag, Robert H.; Sütterlin, Peter; Rutten, Robert J.; Jägers, Aswin P.; Snik, Frans Bibcode: 2004SPIE.5489..362B Altcode: The Dutch Open Telescope (DOT; http://dot.astro.uu.nl) on La Palma is a revolutionary open solar telescope, on an excellent site, on top of a transparent steel tower, and uses natural air flow to minimize local seeing. The aim is long-duration high-resolution imaging with a multi-wavelength camera system. In order to achieve this, the DOT is equipped with a diffraction limited imaging system and uses the speckle reconstruction technique for removing the remaining atmospheric turbulence. The DOT optical system is simple and consists currently of a 0.45m/F4.44 parabolic mirror and a 10x enlargement lens system. We present our plans to increase the aperture of the DOT from 0.45m to 1.4m. The mirror support and telescope top shall be redesigned, but telescope, tower, multi-wavelength camera system and speckle system remain intact. The new optical design permits user selectable choice between angular resolution and field size, as well as transversal pupil shift introducing the possibility to use obstruction free apertures up to 65cm. The design will include a low order AO system, which improves the speckle S/N substantially during moderate seeing conditions. Title: Asymmetrical appearance of dark-cored filaments in sunspot penumbrae Authors: Sütterlin, P.; Bellot Rubio, L. R.; Schlichenmaier, R. Bibcode: 2004A&A...424.1049S Altcode: Recent sunspot observations at unprecedented resolution have led to the discovery of dark cores in the bright filaments that form the penumbra (\citealt{scharmer02_Nat420}). The discovery paper considered spots at disk center only, so the properties of the dark-cored filaments remain largely unknown. Here we analyze a speckle-reconstructed time series of G-band and blue continuum images of a sunspot acquired with the Dutch Open Telescope. The target was located at an heliocentric angle of 27 deg. We confirm the existence of dark-cored penumbral filaments also in spots outside the disk center, and report on distinct differences between the center and limb-side penumbra. In the inner center-side penumbra, filaments are detected as two narrow bright streaks separated by a central obscuration. These structures move together as a single entity. On the limb side, dark cores are hardly seen. The time series is used to determine the sizes (∼200-250 km), proper motions (∼280 m s-1), and lifetimes (⪉45 min) of typical dark-cored filaments. Title: DOT tomography of the solar atmosphere. II. Reversed granulation in Ca II H Authors: Rutten, R. J.; de Wijn, A. G.; Sütterlin, P. Bibcode: 2004A&A...416..333R Altcode: High-quality simultaneous image sequences from the Dutch Open Telescope (DOT) in the G band and the Ca II H line are used to quantify the occurrence of reversed granulation as a constituent of the subsonic brightness pattern observed as a background to acoustic oscillations in the quiet-Sun internetwork atmosphere. In the middle photosphere reversed granulation constitutes a much larger part of this background than at the larger heights sampled by ultraviolet radiation. The anticorrelation with the underlying granulation reaches about 50% at a temporal delay of 2-3 min, and increases with spatial image smoothing to mesogranular resolution. We discuss the nature of reversed granulation in terms of convection reversal, gravity waves, acoustic waves, and intergranular magnetism, suggest that the internetwork background pattern is primarily a mixture of the first two ingredients, and speculate that it is also an inverse canopy mapper. Title: The Dutch Open Telescope on La Palma Authors: Rutten, R. J.; Bettonvil, F. C. M.; Hammerschlag, R. H.; Jägers, A. P. L.; Leenaarts, J.; Snik, F.; Sütterlin, P.; Tziotziou, K.; de Wijn, A. G. Bibcode: 2004IAUS..223..597R Altcode: 2005IAUS..223..597R The Dutch Open Telescope (DOT) on La Palma is an innovative solar telescope combining open telescope structure and an open support tower with a multi-wavelength imaging assembly and with synchronous speckle cameras to generate high-resolution movies which sample different layers of the solar atmosphere simultaneously and co-spatially at high resolution over long durations. The DOT test and development phase is nearly concluded. The installation of an advanced speckle processor enables full science utilization including "Open-DOT" time allocation to the international community. Co-pointing with spectropolarimeters at other Canary Island telescopes and with TRACE furnishes valuable Solar-B precursor capabilities. Title: DOT tomography of the solar atmosphere. I. Telescope summary and program definition Authors: Rutten, R. J.; Hammerschlag, R. H.; Bettonvil, F. C. M.; Sütterlin, P.; de Wijn, A. G. Bibcode: 2004A&A...413.1183R Altcode: The Dutch Open Telescope (DOT) on La Palma is an innovative optical solar telescope capable of reaching 0.2 arcsec angular resolution over extended durations. The DOT presently progresses from technology testbed to a stable science configuration providing multi-wavelength imaging and multi-camera speckle data acquisition for tomographic mapping of the solar atmosphere. Large-volume speckle processing will soon enable frequent usage and community-wide time allocation, in particular for tandem operation with other solar telescopes pursuing spectropolarimetry and EUV imaging. We summarize the DOT hardware and software in the context of this increasing availability and outline the corresponding ``open-DOT'' program. Title: La Palma observations of umbral flashes Authors: Rouppe van der Voort, L. H. M.; Rutten, R. J.; Sütterlin, P.; Sloover, P. J.; Krijger, J. M. Bibcode: 2003A&A...403..277R Altcode: We present high-quality Ca II H & K data showing chromospheric flashes in sunspot umbrae collected with the Swedish Vacuum Solar Telescope, the Dutch Open Telescope, and the Swedish 1-m Solar Telescope at the Roque de los Muchachos Observatory on La Palma. Differential movies, time slices, spectrograms, and Fourier power maps demonstrate that umbral flashes and running penumbral waves are closely related oscillatory phenomena, combining upward shock propagation with coherent wave spreading over the entire spot. We attribute the flash brightening to large redshift by post-shock material higher up. We find no obvious relation between umbral dots and umbral flashes. Title: Motions of Isolated G-Band Bright Points in the Solar Photosphere Authors: Nisenson, P.; van Ballegooijen, A. A.; de Wijn, A. G.; Sütterlin, P. Bibcode: 2003ApJ...587..458N Altcode: 2002astro.ph.12306N Magnetic elements on the quiet Sun are buffeted by convective flows that cause lateral motions on timescales of minutes. The magnetic elements can be observed as bright points (BPs) in the G band at 4305 Å. We present observations of BPs based on a long sequence of G-band images recorded with the Dutch Open Telescope and postprocessed using speckle-masking techniques. From these images we measured the proper motions of isolated BPs and derived the autocorrelation function of their velocity relative to the solar granulation pattern. The accuracy of BP position measurements is estimated to be less than 23 km on the Sun. The rms velocity of BPs (corrected for measurement errors) is about 0.89 km s-1, and the correlation time of BP motions is about 60 s. This rms velocity is about 3 times the velocity measured using cork tracking, almost certainly due to the fact that isolated BPs move more rapidly than clusters of BPs. We also searched for evidence of vorticity in the motions of G-band BPs. Title: Multi-wavelength imaging system for the Dutch Open Telescope Authors: Bettonvil, Felix C.; Suetterlin, Peter; Hammerschlag, Robert H.; Jagers, Aswin P.; Rutten, Robert J. Bibcode: 2003SPIE.4853..306B Altcode: The Dutch Open Telescope (DOT) is an innovative solar telescope, completely open, on an open steel tower, without a vacuum system. The aim is long-duration high resolution imaging and in order to achieve this the DOT is equipped with a diffraction limited imaging system in combination with a data acquisition system designed for use with the speckle masking reconstruction technique for removing atmospheric aberrations. Currently the DOT is being equipped with a multi-wavelength system forming a high-resolution tomographic imager of magnetic fine structure, topology and dynamics in the photosphere and low- and high chromosphere. Finally the system will contain 6 channels: G-band (430.5 nm), Ca II H (K) (396.8 nm), H-α (656.3 nm), Ba II (455.4 nm), and two continuum channels (432 and 651 nm). Two channels are in full operation now and observations show that the DOT produces real diffraction limited movies (with 0.2" resolution) over hours in G-band (430.5 nm) and continuum (432 nm). Title: Dutch Open Telescope: status, results, prospects Authors: Rutten, Robert J.; Sütterlin, Peter; de Wijn, Alfred G.; Hammerschlag, Robert H.; Bettonvil, Felix C. M.; Hoogendoorn, Piet W.; Jägers, Aswin P. L. Bibcode: 2002ESASP.506..903R Altcode: 2002svco.conf..903R; 2002ESPM...10..903R The Dutch Open Telescope (DOT) on La Palma is a revolutionary telescope achieving high-resolution imaging of the solar surface. The DOT combines a pioneering open design at an excellent wind-swept site with image restoration through speckle interferometry. Its open principle is now followed in major solar-telescope projects elsewhere. In the past three years the DOT became the first solar telescope to regularly obtain 0.2" resolution in extended image sequences, i.e., reaching the diffraction limit of its 45-cm primary mirror. Our aim for 2003-2005 is to turn the DOT into a 0.2" tomographic mapper of the solar atmosphere with frequent partnership in international multi-telescope campaigns through student-serviced time allocation. After 2005 we aim to triple the DOT resolution to 0.07" by increasing the aperture to 140 cm and to renew the speckle cameras and the speckle pipeline in order to increase the field size and sequence duration appreciably. These upgrades will maintain the DOT's niche as a tomographic high-resolution mapper in the era when GREGOR, Solar-B and SDO set the stage. Title: Opening the Dutch Open Telescope Authors: Rutten, R. J.; de Wijn, A. G.; Sütterlin, P.; Bettonvil, F. C. M.; Hammerschlag, R. H. Bibcode: 2002ESASP.505..565R Altcode: 2002IAUCo.188..565R; 2002solm.conf..565R We hope to "open the DOT" to the international solar physics community as a facility for high-resolution tomography of the solar atmosphere. Our aim is to do so combining peer-review time allocation with service-mode operation in a "hands-on-telescope" education program bringing students to La Palma to assist in the observing and processing. The largest step needed is considerable speedup of the DOT speckle processing. Title: THEMIS and DOT joint observations on NOAA 9716 Authors: Briand, C.; Collados, M.; Sütterlin, P. Bibcode: 2002ESASP.505..361B Altcode: 2002IAUCo.188..361B; 2002solm.conf..361B Ephemeral magnetic emergence has been detected in a decaying β region observed in December 2001 simultaneously with the DOT and THEMIS. We present here the main characteristics of this phenomenon. Also the time evolution of a small group of pores is shown together with the time evolution of an horizontal magnetic field overlying them. Title: Fine structure in sunspots. IV. Penumbral grains in speckle reconstructed images Authors: Sobotka, M.; Sütterlin, P. Bibcode: 2001A&A...380..714S Altcode: The properties of penumbral grains (PGs) in a large regular sunspot are studied from a 70 min sequence of G-band images acquired on 20 September 1999 at the Dutch Open Telescope, La Palma. The frames were processed using the speckle masking algorithm, resulting in an almost diffraction-limited time series (30 s cadence), basically free of atmospheric distortions. Applying feature tracking to a movie of 140 frames yields proper motions, intensities, and lifetimes for a set of 1058 PGs with lifetimes longer than 10 min. About 54% of the PGs move toward the umbra and 46% toward the photosphere. The inward-moving PGs are located mostly in the inner penumbra (up to 0.6 of the distance from the umbra to the photosphere). Their average lifetime and median speed are 50 min and 0.52 km s-1. Most of the outward-moving PGs are observed in the outer penumbra and their average lifetime and median speed are 31 min and 0.75 km s-1. These measurements confirm the previous results published by Sobotka et al. (\cite{sobotka99_AA348}). Title: Penumbral finestructure: need for larger telescopes Authors: Balthasar, H.; Sütterlin, P.; Collados, M. Bibcode: 2001AN....322..367B Altcode: We obtained at the same time G-band images at the Dutch Open Telescope (DOT) on La Palma and spectropolarimetric data in the near infrared at the German Vacuum Tower Telescope (VTT) on Tenerife. The spectropolarimetric data show interesting correlations. Bright filaments have a smaller magnetic field s trength, and higher Evershed velocities occur in dark structures. This result is in agreement with some previous observations, but also in contradiction to others. However, we suffer from the fact that the resolution limit of the VTT at a wavelength of 1.565 μm corresponds to 400 km. Spatial power spectra derived from the DOT data indicate a typical width of 250 km for penumbral filaments. Obviously a solar telescope with an aperture of at least 1.5~m is needed to obtain sophisticated results for penumbral structures. Title: Ba II 4554 Å speckle imaging as solar Doppler diagnostic Authors: Sütterlin, P.; Rutten, R. J.; Skomorovsky, V. I. Bibcode: 2001A&A...378..251S Altcode: We present observations testing the Dopplergram capability of a narrow-band (80 mÅ) Lyot filter imaging the solar surface in the wings of the Ba II 4554 Å resonance line in combination with speckle reconstruction to obtain high angular resolution. The Ba II 4554 Å line is found to be an excellent tool for high-resolution Doppler mapping thanks to opacity insensitivity to temperature variations and line-width insensitivity to thermal broadening. The resulting Dopplergrams show concentrated downflows of 1.2-2.2 km;s-1 in intergranular lanes that probably mark magnetic fluxtubes. Two-wavelength profile sampling is found to suffice for high-resolution Dopplergram construction. The filter will be installed as part of a multi-wavelength speckle imaging system on the new Dutch Open Telescope. Title: The size of penumbral fine structure Authors: Sütterlin, P. Bibcode: 2001A&A...374L..21S Altcode: I present power spectra of penumbral and granular intensity variations from a speckle-restored G-band image sequence of sunspot NOAA 9407 taken on April 1, 2001 with the Dutch Open Telescope on La Palma. I compare spatial power spectra of the sunspot penumbra with spatial power spectra from granulation with and without filigree. Relative to the granular power distribution, the penumbral power spectrum is enhanced over a wide range in spatial frequency peaking at 0\farcs35. For smaller scales, the penumbral power distribution closely resembles that of the granular intensity variations. In contrast, the power spectrum of granulation with filigree exhibits increased power down to the resolution limit of 0\farcs22, indicating the presence of unresolved magnetic elements. Title: Active Region Oscillations: Results from SOHO JOP 097 Authors: O'Shea, E.; Fleck, B.; Muglach, K.; Sütterlin, P. Bibcode: 2001AGUSM..SH41A02O Altcode: We present here an analysis of data obtained in a sunspot region, using the Coronal Diagnostic Spectrometer (CDS) on SOHO. These data were obtained in the context of the Joint Observing Program (JOP) 97 which, together with CDS, included the Michelson Doppler Imaging (MDI) instrument on SOHO, the TRACE satellite and various ground based observatories, e.g. the DOT on La Palma. Using the lines of Fe XVI 335, Mg IX 368, He I 584, O III 599, Mg X 624 and O V 624 of CDS time series data were obtained in the pore and plage regions of sunspots associated with active regions AR 9166, 9166 and 9169 between September 19-29 2000. In addition to the time series datasets we also obtained 240 arcsec x 240 arcsec raster images of the sunspot regions examined. Using different time series analysis techniques we analyse the different periods of oscillation found in time series datasets and present the results here. This research is part of the European Solar Magnetometry Network supported by the EC through the TMR programme. Title: Penumbral Finestructure: Need for Larger Telescopes Authors: Balthasar, H.; Sütterlin, P.; Collados, M. Bibcode: 2001AGM....18S1003B Altcode: We obtained at the same time G-band images at the Dutch Open Telescope (DOT) on La Palma and spectropolarimetric data in the near infrared at the German Vacuum Tower Telescope (VTT) on Tenerife. The spectropolarimetric data show interesting correlations. Bright filaments have a smaller magnetic field strength, and higher Evershed velocities occur in dark structures. This result is in agreement with some previous observations, but also in contradiction to others. However, we suffer from the fact that the resolution limit of the VTT at a wavelength of 1.565 μm corresponding to 400 km. Spatial power spectra derived from the DOT data indicate a typical width of 250 km for the penumbral filaments. Obviously a solar telescope with an aperture of at least 1.5 m is needed to obtain sophisticated results for penumbral structures. Title: Proxy Magnetometry with the Dutch Open Telescope Authors: Rutten, R. J.; Hammerschlag, R. H.; Sütterlin, P.; Bettonvil, F. C. M. Bibcode: 2001ASPC..236...25R Altcode: 2001aspt.conf...25R No abstract at ADS Title: A Multi-Channel Speckle Imaging System for the DOT Authors: Sütterlin, P.; Hammerschlag, R. H.; Bettonvil, F. C. M.; Rutten, R. J.; Skomorovsky, V. I.; Domyshev, G. N. Bibcode: 2001ASPC..236..431S Altcode: 2001aspt.conf..431S No abstract at ADS Title: Dutch Open Telescope: Status and Prospects Authors: Rutten, R. J.; Hammerschlag, R. H.; Bettonvil, F. M.; Suetterlin, P. Bibcode: 2000SPD....3102107R Altcode: 2000BAAS...32.1290R The Dutch Open Telescope (DOT) on La Palma in the Canary Islands is a small but revolutionary solar telescope of which the image quality matches the superb imaging of the Swedish Vacuum Solar Telescope (from whose building the DOT is operated). The DOT is an open parabolic 45cm reflector on an open 15m high tower, relying on mirror flushing by the trade winds that bring the best seeing at La Palma to avoid internal turbulence. A water-cooled field stop in the primary image reflects most sunlight and heat out of the telescope. The first data from the DOT combined with speckle reconstruction have yielded sunspot movies of outstanding quality. At present, a multi-channel imaging system is in construction for simultaneous registration of speckle sequences in the G band, in Ca II K and in Hα. The data pipeline permits continuous speckle data acquisition up to 0.5 Tb per day. The advantage of speckle reconstruction over adaptive optics is the much larger field of the restored scene, with the DOT camera's 100x130 arcsec at 0.2 arcsec resolution. The DOT science program is to study magnetic topology and dynamics throughout the photosphere and chromosphere. Title: Applying speckle masking to spectra Authors: Sütterlin, P.; Wiehr, E. Bibcode: 2000SoPh..194...35S Altcode: We have applied the technique of speckle masking to spectra. The observation of elongated solar structures avoids the problem of missing information in one-dimensional spectra. Image motion perpendicular to the slit was diminished by a one-dimensional image stabilization system. The remaining influence of the Earth's atmosphere was removed by a modified speckle-masking algorithm, adapted to the single spatial dimension occurring in the spectra. The reconstructed spectra achieve the diffraction limit of the telescope and the spectrograph. Title: Solar Magnetometry with the Dutch Open Telescope Authors: Rutten, R.; Hammerschlag, R. H.; Sutterlin, P.; Bettonvil, F. C. M.; van der Zalm, E. B. J. Bibcode: 2000ESASP.463..611R Altcode: 2000sctc.proc..611R No abstract at ADS Title: Continuum photometry of solar white-light faculae Authors: Sütterlin, P.; Wiehr, E.; Stellmacher, G. Bibcode: 1999SoPh..189...57S Altcode: We have determined absolute continuum intensities and brightness temperatures of individual facular grains at a spatial resolution limited by the φ=50 cm aperture of the SVST on La Palma. A facular region at θ≈57° was observed simultaneously in three narrow continuum windows at 450.5, 658.7, and 863.5 nm. We corrected for image degradation by the Earth's atmosphere using the speckle masking method. The brightness temperatures do not exactly follow the Planck law. The differences of Tblue−Tred=220 K and Tir−Tred=−42 K reflect the wavelength dependence of the continuum formation depth. The (red) temperatures of 250 facular grains show excesses between 250 and 450 K above their undisturbed neighborhood. The wavelength dependence of the relative intensity ratios Cλ= [Ifac/Iphot] λshow a large scatter around mean values of Cblue/Cred=1.075 and Cir/Cred=0.98. We determined the center-to-limb variation of the 863.5 nm continuum contrast for 0.17>cosθ>0.39 by measuring 270 grains in reconstructed facular images. The upper envelope of the data points increases linearly to 1.5 at cos θ=0.17. Application of the mean color dependence yields green contrasts up to C550=1.7, which is far higher than previously observed values. The behaviour for cos θ>0.17 is estimated from (unreconstructed) frame-selected best images taken over a time interval of 7 hours. Six distinct facular regions clearly discernible during the whole time interval indicate a slight contrast decrease towards the extreme limb. The observed quantities are useful for an adjustment of model calculations and for a discrimination of competing models. Title: Automatic guiding of the primary image of solar Gregory telescopes Authors: Küveler, G.; Wiehr, E.; Thomas, D.; Harzer, M.; Bianda, M.; Epple, A.; Sütterlin, P.; Weisshaar, E. Bibcode: 1998SoPh..182..247K Altcode: The primary image reflected from the field stop of solar Gregory telescopes is used for automatic guiding. This new system avoids temporal varying influences from the bending of the telescope tube by the main mirror's gravity and from offsets between the telescope and a separate guiding refractor. The required stiffness of the guider mechanics and the small areas of the sensors demand small f numbers for the guider optics, which cause problems with the image quality and with heat. Problems also arise from the pinhole in the telescope's field stop which is imaged as a dark dot on the sensor. Pointing errors introduced by the telescope affect shifts of the solar image on the sensor. These are numerically determined by Fourier methods which are found to be less sensitive to noise than profile centering methods. Several types of guiders are tested, the final equipment, now installed at the Gregory telescopes at Tenerife and at Locarno, is described. Title: Temperature mapping of sunspots and pores from speckle reconstructed three colour photometry Authors: Suetterlin, P.; Wiehr, E. Bibcode: 1998A&A...336..367S Altcode: The two-dimensional temperature distribution in a highly structured sunspot and in two small umbrae is determined from a three-colour photometry in narrow spectral continua. Disturbing influences from the earth's atmosphere are removed by speckle masking techniques, yielding a spatial resolution limited by the telescope's aperture. The corresponding colour temperatures are consistent over a range of more than 2000 K, although the numerical correction introduced by the reconstruction differs largely for the three colours. Part of the scatter in the temperature relation disappears when convoluting the final images with artificial PSFs that compensate for the different, colour dependent spatial resolution. The remaining spread in the scatter plots does not reflect noise, but is related to local variations of the temperature difference between the continuum emitting layers. This is most obvious for a small umbra which yields `branches' in the scatter plots the `bluer' of which corresponding to the limb-side umbral border. Here, the `hot rim' of a Wilson depressed umbra becomes visible. The temperature map of the large spot shows that the bright umbral dots do not reach the temperature of the non-spot surroundings. Instead, they exceed the 2000 K cooler umbral temperature minimum by 900-1300 K. The filamentary structure of the surrounding penumbra has spatial temperature fluctuations of typically 700 K, a value which fits earlier observed contrasts. However, the mean temperatures of 5650 K in the dark and 6250 K in the bright penumbral fine structures exceed former findings. Exceptionally bright penumbral grains are 250 K hotter than the mean solar surface and thus exceed even brightest granules. Title: Properties of solar pores Authors: Suetterlin, Peter Bibcode: 1998A&A...333..305S Altcode: We present the results of an extensive investigation of the properties of solar pores. Spectra of all 4 Stokes parameters of several magnetic sensitive absorption lines as well as Stokes ( I ) only spectra of lines with low or vanishing Lande-factor have been observed. An inversion code based on the Levenberg-Marquardt algorithm was written, incorporating the full radiative transfer equations. This code adapts a set of parameters that are used to represent the magnetic and thermodynamic state of the atmosphere to best-fit a set of observed line profiles. The results show that the value of all relevant parameters are intimately related to the size of the pores, therewith confirming the role of solar pores as a link between (bright) magnetic flux tubes and sunspots. In particular the inclination of the magnetic field lines at the outer rim of a pore, which is affected by the vertical field gradient, seems to put an upper limit on the diameter of pores where the inclination reaches a value that might trigger the formation of a penumbra. Title: Simultaneous Observations with the GCT and SoHO: High Velocity Events in the Upper Chromosphere Authors: Muglach, K.; Sütterlin, P. Bibcode: 1998ASPC..155..341M Altcode: 1998sasp.conf..341M No abstract at ADS Title: 3-Color Photometry of a Sunspot Using Speckle Masking Techniques Authors: Sütterlin, P.; Wiehr, E. Bibcode: 1998ASPC..155...44S Altcode: 1998sasp.conf...44S No abstract at ADS Title: Prominence Emissions with SUMER and Optical Telescopes Authors: Wiehr, E.; Stellmacher, G.; de Boer, C. R.; Sütterlin, P. Bibcode: 1998ASPC..155..331W Altcode: 1998sasp.conf..331W No abstract at ADS Title: Problems in measuring prominence oscillations. Authors: Suetterlin, P.; Wiehr, E.; Bianda, M.; Kueveler, G. Bibcode: 1997A&A...321..921S Altcode: Time variations of Doppler shifts of the Ca^+^ 8542Å emission in quiescent solar prominences have been measured. A new type of 'limb guider' assures a highly constant distance of the spectrograph slit from the solar limb and furthermore removes low frequency image motion in the direction perpendicular to the slit. Remaining image motion along the slit is usually removed by a correlation of subsequent spectra. This procedure, however, cannot be applied globally to the whole spatial height in the spectra if individual structures exhibit lateral motions along the slit or even decay or arise during the observation. We therefore correlate defined individual emission maxima from successive spectra. The finally obtained power spectra show oscillations with a variety of periods at restricted locations. The data favour the known general presence of periods near 20 and 60-min, however they give only slight indication for `typical' periods near 3 and 5 min. Title: Three colour photometry of a sunspot using speckle masking techniques. Authors: Sütterlin, P.; Wiehr, E. Bibcode: 1997AGAb...13...10S Altcode: No abstract at ADS Title: Emission Lines in a Quiescent Prominence Observed with the SUMER, VTT, a nd Gregory Telescopes Authors: Wiehr, E.; de Boer, C. R.; Stellmacher, G.; Sutterlin, P. Bibcode: 1997ASPC..118..294W Altcode: 1997fasp.conf..294W A number of emission lines have been observed in a quiescent prominence with the two German telescopes on Tenerife and with SUMER. The Gregory telescope monitored simultaneously the lines H_8 3889, He 3888, Ca(+\:) 8498, the VTT simultaneously the lines H_β, He-D_3, Ca(+\:) 8542, and H_α slit jaw pictures on 1024 x 1024 CCDs. At the same time, SUMER monitored successively three combinations of EUV lines containing He 584, He 537 in the second, and Ar, N(+) , C, C(++) , O, O(+) , O(++) , S(++) , S(+++) lines in the first order. Two dimensional images in the various EUV lines establish the anti-relation between temperature and Balmer brightness. Title: Polarimetry of Solar Pores Authors: Sütterlin, P.; Schröter, E. H.; Muglach, K. Bibcode: 1996SoPh..164..311S Altcode: We address the magnetic field structure of solar pores. The data were obtained at the Gregory Coudè telescope at Izaña using the AT1 CCD camera system to observe pores with three spectral lines: one magnetically sensitive line, recording all 4 Stokes profiles, and two g = 0 lines where only the intensity profiles were measured. The data reduction included the standard procedure (removing dark current and flatfielding) as well as destretching of the polarimetric spectra and removing the non-magnetic straylight by means of a 2-d deconvolution of the observed intensity variation using a Lucy-Richardson restoration algorithm. In the following analysis we first determined the temperature- and pressure stratification of the pore using the g = 0 lines and then applied an inversion of the Stokes profiles to get the parameters of the magnetic field. Across the pore we find a strong variation of the resulting field strength as well as of the inclination and the azimuth, consistent with the assumption of a canopy forming in the higher atmosphere. Title: Über die Struktur von solaren Poren Title: Über die Struktur von solaren Poren Title: On the structure of solar pores; Authors: Sütterlin, Peter Pit Bibcode: 1996PhDT.......153S Altcode: No abstract at ADS Title: Neue Verarbeitungstechniken für CCD-Bilder. Einige Anregungen. Authors: Sütterlin, P. Bibcode: 1994S&W....33..311S Altcode: No abstract at ADS Title: On the magneto-thermodynamical structure of a solar pore Authors: Sütterlin, P.; Thim, F.; Schröter, E. H. Bibcode: 1994smf..conf..213S Altcode: No abstract at ADS Title: Erfahrungen mit CCD-Kameras in der Amateurastronomie. Authors: Sütterlin, P. Bibcode: 1994aks..1995Q....S Altcode: No abstract at ADS