Author name code: loefdahl ADS astronomy entries on 2022-09-14 author:"Loefdahl, Mats" ------------------------------------------------------------------------ Title: The European Solar Telescope Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.; Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein, C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados, M.; the EST team Bibcode: 2022arXiv220710905Q Altcode: The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope (DOT). With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems. Title: Multi-frame blind deconvolution and phase diversity with statistical inclusion of uncorrected high-order modes Authors: Löfdahl, Mats G.; Hillberg, Tomas Bibcode: 2022arXiv220513650L Altcode: Images collected with ground-based telescopes suffer blurring and distortions from turbulence in Earth's atmosphere. Adaptive optics (AO) can only partially compensate for these effects. Neither multi-frame blind deconvolution (MFBD) nor speckle techniques restore AO compensated images to the correct power spectrum and contrast. MFBD can only compensate for a finite number of low-order aberrations, leaving a tail of uncorrected high-order modes. Speckle restoration of AO-corrected data depends on calibrations of the AO corrections and assumptions regarding the height distribution of atmospheric turbulence. We seek to develop an improvement to MFBD that combines speckle's usage of turbulence statistics to account for high-order modes with the ability of MFBD to sense low-order modes that can be partially corrected by AO and/or include fixed or slowly changing instrumental aberrations. We modify the image formation model, supplementing the fitted low-order wavefront aberrations with tails of random high-order aberrations that follow Kolmogorov statistics, scaled to estimated or measured values of Fried's parameter, r0, that characterize the strength of the seeing at the moment of data collection. We refer to this as statistical diversity (SD). We test MFBD with SD with noise-free synthetic data, simulating many different r0 and numbers of AO-corrected modes. SD improves the contrasts and power spectra of restored images, both in accuracy and in consistency with varying r0, without penalty in processing time. With focus diversity (FD), the results are almost perfect. SD also reduces errors in the fitted wavefront parameters. MFBD with SD and FD seems robust with respect to several percents of error in r0. Adding SD to MFBD shows great promise for improving contrasts and power spectra in restored images. Further studies with real data are motivated. Title: SSTRED: Data- and metadata-processing pipeline for CHROMIS and CRISP Authors: Löfdahl, Mats G.; Hillberg, Tomas; de la Cruz Rodríguez, Jaime; Vissers, Gregal; Andriienko, Oleksii; Scharmer, Göran B.; Haugan, Stein V. H.; Fredvik, Terje Bibcode: 2021A&A...653A..68L Altcode: 2018arXiv180403030L Context. Data from ground-based, high-resolution solar telescopes can only be used for science with calibrations and processing, which requires detailed knowledge about the instrumentation. Space-based solar telescopes provide science-ready data, which are easier to work with for researchers whose expertise is in the interpretation of data. Recently, data-processing pipelines for ground-based instruments have been constructed.
Aims: We aim to provide observers with a user-friendly data pipeline for data from the Swedish 1-meter Solar Telescope (SST) that delivers science-ready data together with the metadata needed for proper interpretation and archiving.
Methods: We briefly describe the CHROMospheric Imaging Spectrometer (CHROMIS) instrument, including its (pre)filters, as well as recent upgrades to the CRisp Imaging SpectroPolarimeter (CRISP) prefilters and polarization optics. We summarize the processing steps from raw data to science-ready data cubes in FITS files. We report calibrations and compensations for data imperfections in detail. Misalignment of Ca II data due to wavelength-dependent dispersion is identified, characterized, and compensated for. We describe intensity calibrations that remove or reduce the effects of filter transmission profiles as well as solar elevation changes. We present REDUX, a new version of the MOMFBD image restoration code, with multiple enhancements and new features. It uses projective transforms for the registration of multiple detectors. We describe how image restoration is used with CRISP and CHROMIS data. The science-ready output is delivered in FITS files, with metadata compliant with the SOLARNET recommendations. Data cube coordinates are specified within the World Coordinate System (WCS). Cavity errors are specified as distortions of the WCS wavelength coordinate with an extension of existing WCS notation. We establish notation for specifying the reference system for Stokes vectors with reference to WCS coordinate directions. The CRIsp SPectral EXplorer (CRISPEX) data-cube browser has been extended to accept SSTRED output and to take advantage of the SOLARNET metadata.
Results: SSTRED is a mature data-processing pipeline for imaging instruments, developed and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP spectropolarimeter. SSTRED delivers well-characterized, science-ready, archival-quality FITS files with well-defined metadata. The SSTRED code, as well as REDUX and CRISPEX, is freely available through git repositories. Title: Is the sky the limit?. Performance of the revamped Swedish 1-m Solar Telescope and its blue- and red-beam reimaging systems Authors: Scharmer, G. B.; Löfdahl, M. G.; Sliepen, G.; de la Cruz Rodríguez, J. Bibcode: 2019A&A...626A..55S Altcode: 2019arXiv190505588S We discuss the use of measurements of the solar granulation contrast as a measure of optical quality. We demonstrate that for data recorded with a telescope that uses adaptive optics and/or post-processing to compensate for many low- and high-order aberrations, the RMS granulation contrast is directly proportional to the Strehl ratio calculated from the residual (small-scale) wavefront error (static and/or from seeing). We demonstrate that the wings of the high-order compensated point spread function for the Swedish 1-m Solar Telescope (SST) are likely to extend to a radius of not more than about 2″, which is consistent with earlier conclusions drawn from stray-light compensation of sunspot images. We report on simultaneous measurements of seeing and solar granulation contrast averaged over 2 s time intervals at several wavelengths from 525 nm to 853.6 nm on the red-beam (CRISP beam) and wavelengths from 395 nm to 484 nm on the blue-beam (CHROMIS beam). These data were recorded with the SST, which has been revamped with an 85-electrode adaptive mirror and a new tip-tilt mirror, both of which were polished to exceptionally high optical quality. Compared to similar data obtained with the previous 37-electrode adaptive mirror in 2009 and 2011, there is a significant improvement in image contrast. The highest 2 s average image contrasts measured in April 2015 through 0.3-0.9 nm interference filters at 525 nm, 557 nm, 630 nm, and 853.5 nm with compensation only for the diffraction limited point spread function of SST are 11.8%, 11.8%, 10.2%, and 7.2%, respectively. Similarly, the highest 2 s contrasts measured at 395 nm, 400 nm, and 484 nm in May 2016 through 0.37-1.3 nm filters are 16%, 16%, and 12.5%, respectively. The granulation contrast observed with SST compares favorably to measured values with SOT on Hinode and with Sunrise as well as major ground-based solar telescopes. Simultaneously with the above wideband red-beam data, we also recorded narrowband continuum images with the CRISP imaging spectropolarimeter. We find that contrasts measured with CRISP are entirely consistent with the corresponding wideband contrasts, demonstrating that any additional image degradation by the CRISP etalons and telecentric optical system is marginal or even insignificant. Finally, we discuss the origin of the 48 nm RMS wavefront error needed to bring consistency between the measured granulation contrast and that obtained from 3D simulations of convection. Title: Mapping the Magnetic Field of Flare Coronal Loops Authors: Kuridze, D.; Mathioudakis, M.; Morgan, H.; Oliver, R.; Kleint, L.; Zaqarashvili, T. V.; Reid, A.; Koza, J.; Löfdahl, M. G.; Hillberg, T.; Kukhianidze, V.; Hanslmeier, A. Bibcode: 2019ApJ...874..126K Altcode: 2019arXiv190207514K Here, we report on the unique observation of flaring coronal loops at the solar limb using high-resolution imaging spectropolarimetry from the Swedish 1 m Solar Telescope. The vantage position, orientation, and nature of the chromospheric material that filled the flare loops allowed us to determine their magnetic field with unprecedented accuracy using the weak-field approximation method. Our analysis reveals coronal magnetic field strengths as high as 350 G at heights up to 25 Mm above the solar limb. These measurements are substantially higher than a number of previous estimates and may have considerable implications for our current understanding of the extended solar atmosphere. 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: A Comparison of Solar Image Restoration Techniques for SST/CRISP Data (Summary) Authors: Löfdahl, M. Bibcode: 2016ASPC..504..111L Altcode: Solar images from high-resolution, ground-based telescopes are corrected for the blurring effects of atmospheric turbulence by use of adaptive optics and post-facto image restoration. Two classes of image restoration methods are regularly used today, those based on Multi-Frame Blind Deconvolution (MFBD; Löfdahl 2002) and those based on Speckle Interferometry (SI; von der Luhe &Dunn 1987). In a recently started project, we will compare and evaluate such methods for use with spectropolarimetric data from the CRisp Imaging SpectroPolarimeter (CRISP; Scharmer et al. 2008) of the Swedish 1-meter Solar Telescope (SST; Scharmer et al. 2003). For SST/CRISP data we routinely use the Multi-Object MFBD (MOMFBD; van Noort et al. 2005) technique to jointly restore images collected from a wideband camera and from the narrowband cameras behind the CRISP FPI and polarimetry optics. This crucial step in the data reduction pipeline of CRISP (CRISPRED; de la Cruz Rodríguez et al. 2015) is carefully integrated with the application of various procedures that are designed to reduce effects of imperfections in the instruments. In order to make the comparison as fair as possible, we have extended CRISPRED so that the Kiepenheuer-Institut Speckle Interferometry Package (KISIP; Wöger & von der Lühe 2008), together with Speckle Deconvolution (SD; Keller & von der Luehe 1992; Mikurda et al. 2006), can serve as a drop in replacement for MOMFBD. The adaption of SI and SD to CRISPRED will allow us to make fair comparisons not only of the restored images, but also of derivative data like Stokes maps and further on to evaluate the consequences of remaining errors and artifacts for the interpretation of physical quantities inferred through atmospheric model inversions. Title: Off-disk straylight measurements for the Swedish 1-m Solar Telescope Authors: Löfdahl, Mats G. Bibcode: 2016A&A...585A.140L Altcode: 2015arXiv151008329L Context. Accurate photometry with ground-based solar telescopes requires characterization of straylight. Scattering in Earth's atmosphere and in the telescope optics are potentially significant sources of straylight, for which the point spread function (PSF) has wings that reach very far. This kind of straylight produces an aureola, extending several solar radii off the solar disk.
Aims: We want to measure such straylight using the ordinary science instrumentation.
Methods: We scanned the intensity on and far off the solar disk by use of the science cameras in several different wavelength bands on a day with low-dust conditions. We characterized the far wing straylight by fitting a model to the recorded intensities involving a multicomponent straylight PSF and the limb darkening of the disk.
Results: The measured scattered light adds an approximately constant fraction of the local granulation intensity to science images at any position on the disk. The fraction varied over the day but never exceeded a few percent. The PSFs have weak tails that extend to several solar radii, but most of the scattered light originates within ~1'.
Conclusions: Far-wing scattered light contributes only a small amount of straylight in SST data. Other sources of straylight are primarily responsible for the reduced contrast in SST images. 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: Restoration of the contrast in solar images Authors: Loefdahl, M. Bibcode: 2012IAUSS...6E.305L Altcode: Solar images from the major ground-based high-resolution telescopes are routinely processed to compensate for blurring and distortion caused by turbulence in Earth's atmosphere and only partly removed by adaptive optics. When the seeing conditions are good enough, methods based on multi-frame blind deconvolution, phase diversity, and speckle interferometry deliver images with nearly diffraction limited resolution. This corresponds to good estimates of the core of the short-exposure point spread functions (PSFs). Combining many short exposures results in high-resolution images with high signal to noise ratio. This approach has been used successfully e.g., for studies of the velocity and magnetic fields of small-scale photospheric and chromospheric solar features, and for their temporal evolution. However, the contrast and spatial power spectra of ground-based solar images are usually severely under-estimated. We know this from recent advances in space-based observations and MHD modeling of the photosphere, that until recently did not agree on the contrast of solar image data. It is now well established that synthetic data from current codes produce data with contrasts and power spectra that well represent reality. This means it is now possible to test methods for estimating the effects of various sources of reduced contrast. We have recently set out on a search for such sources present at the Swedish 1-m Solar Telescope. In this presentation I will discuss some of our findings and some of our ideas for further testing. Title: Sources of straylight in the post-focus imaging instrumentation of the Swedish 1-m Solar Telescope Authors: Löfdahl, M. G.; Scharmer, G. B. Bibcode: 2012A&A...537A..80L Altcode: 2011arXiv1110.2663L Context. Recently measured straylight point spread functions (PSFs) in Hinode/SOT make granulation contrast in observed data and synthetic magnetohydrodynamic (MHD) data consistent. Data from earthbound telescopes also need accurate correction for straylight and fixed optical aberrations.
Aims: We aim to develop a method for measuring straylight in the post-focus imaging optics of the Swedish 1-m Solar Telescope (SST).
Methods: We removed any influence from atmospheric turbulence and scattering by using an artificial target. We measured integrated straylight from three different sources in the same data: ghost images caused by reflections in the near-detector optics, PSFs corresponding to wavefront aberrations in the optics by using phase diversity, and extended scattering PSF wings of unknown origin by fitting to a number of different kernels. We performed the analysis separately in the red beam and the blue beam.
Results: Wavefront aberrations, which possibly originate in the bimorph mirror of the adaptive optics, are responsible for a wavelength-dependent straylight of 20-30% of the intensity in the form of PSFs with 90% of the energy contained within a radius of 0.6 arcsec. There are ghost images that contribute at the most a few percent of straylight. The fraction of other sources of scattered light from the post-focus instrumentation of the SST is only ~10-3 of the recorded intensity. This contribution has wide wings with a FWHM ~16'' in the blue and ~34'' in the red.
Conclusions: The present method seems to work well for separately estimating wavefront aberrations and the scattering kernel shape and fraction. Ghost images can be expected to remain at the same level for solar observations. The high-order wavefront aberrations possibly caused by the AO bimorph mirror dominate the measured straylight but are likely to change when imaging the Sun. We can therefore make no firm statements about the origin of straylight in SST data, but strongly suspect wavefront aberrations to be the dominant source. Title: A tilted interference filter in a converging beam Authors: Löfdahl, M. G.; Henriques, V. M. J.; Kiselman, D. Bibcode: 2011A&A...533A..82L Altcode: 2011arXiv1108.1234L Context. Narrow-band interference filters can be tuned toward shorter wavelengths by tilting them from the perpendicular to the optical axis. This can be used as a cheap alternative to real tunable filters, such as Fabry-Pérot interferometers and Lyot filters. At the Swedish 1-meter Solar Telescope, such a setup is used to scan through the blue wing of the Ca ii H line. Because the filter is mounted in a converging beam, the incident angle varies over the pupil, which causes a variation of the transmission over the pupil, different for each wavelength within the passband. This causes broadening of the filter transmission profile and degradation of the image quality.
Aims: We want to characterize the properties of our filter, at normal incidence as well as at different tilt angles. Knowing the broadened profile is important for the interpretation of the solar images. Compensating the images for the degrading effects will improve the resolution and remove one source of image contrast degradation. In particular, we need to solve the latter problem for images that are also compensated for blurring caused by atmospheric turbulence.
Methods: We simulate the process of image formation through a tilted interference filter in order to understand the effects. We test the hypothesis that they are separable from the effects of wavefront aberrations for the purpose of image deconvolution. We measure the filter transmission profile and the degrading PSF from calibration data.
Results: We find that the filter transmission profile differs significantly from the specifications. We demonstrate how to compensate for the image-degrading effects. Because the filter tilt effects indeed appear to be separable from wavefront aberrations in a useful way, this can be done in a final deconvolution, after standard image restoration with Multi-Frame Blind Deconvolution/Phase Diversity based methods. We illustrate the technique with real data. Title: Evaluation of image-shift measurement algorithms for solar Shack-Hartmann wavefront sensors Authors: Löfdahl, M. G. Bibcode: 2010A&A...524A..90L Altcode: 2010arXiv1009.3401L Context. Solar Shack-Hartmann wavefront sensors measure differential wavefront tilts as the relative shift between images from different subapertures. There are several methods in use for measuring these shifts.
Aims: We evaluate the inherent accuracy of the methods and the effects of various sources of error, such as noise, bias mismatch, and blurring. We investigate whether Z-tilts or G-tilts are measured.
Methods: We test the algorithms on two kinds of artificial data sets, one corresponding to images with known shifts and one corresponding to seeing with different r0.
Results: Our results show that the best methods for shift measurements are based on the square difference function and the absolute difference function squared, with subpixel accuracy accomplished by use of two-dimensional quadratic interpolation. These methods measure Z-tilts rather than G-tilts. Title: Striation and convection in penumbral filaments Authors: Spruit, H. C.; Scharmer, G. B.; Löfdahl, M. G. Bibcode: 2010A&A...521A..72S Altcode: 2010arXiv1008.0932S Observations with the 1-m Swedish Solar Telescope of the flows seen in penumbral filaments are presented. Time sequences of bright filaments show overturning motions strikingly similar to those seen along the walls of small isolated structures in the active regions. The filaments show outward propagating striations with inclination angles suggesting that they are aligned with the local magnetic field. We interpret it as the equivalent of the striations seen in the walls of small isolated magnetic structures. Their origin is then a corrugation of the boundary between an overturning convective flow inside the filament and the magnetic field wrapping around it. The outward propagation is a combination of a pattern motion due to the downflow observed along the sides of bright filaments, and the Evershed flow. The observed short wavelength of the striation argues against the existence of a dynamically significant horizontal field inside the bright filaments. Its intensity contrast is explained by the same physical effect that causes the dark cores of filaments, light bridges and “canals”. In this way striation represents an important clue to the physics of penumbral structure and its relation with other magnetic structures on the solar surface. We put this in perspective with results from the recent 3-D radiative hydrodynamic simulations.

4 movies are only available in electronic form at http://www.aanda.org Title: High-order aberration compensation with multi-frame blind deconvolution and phase diversity image restoration techniques Authors: Scharmer, G. B.; Löfdahl, M. G.; van Werkhoven, T. I. M.; de la Cruz Rodríguez, J. Bibcode: 2010A&A...521A..68S Altcode: 2010arXiv1007.1236S Context. For accurately measuring intensities and determining magnetic field strengths of small-scale solar (magnetic) structure, knowledge of and compensation for the point spread function is crucial. For images recorded with the Swedish 1-meter Solar Telescope (SST), restoration with multi-frame blind deconvolution (MFBD) and joint phase diverse speckle (JPDS) methods lead to remarkable improvements in image quality but granulation contrasts that are too low, indicating additional stray light.
Aims: We propose a method to compensate for stray light from high-order atmospheric aberrations not included in MFBD and JPDS processing.
Methods: To compensate for uncorrected aberrations, a reformulation of the image restoration process is proposed that allows the average effect of hundreds of high-order modes to be compensated for by relying on Kolmogorov statistics for these modes. The applicability of the method requires simultaneous measurements of Fried's parameter r0. The method is tested with simulations as well as real data and extended to include compensation for conventional stray light.
Results: We find that only part of the reduction of granulation contrast in SST images is due to uncompensated high-order aberrations. The remainder is still unaccounted for and attributed to stray light from the atmosphere, the telescope with its re-imaging system and to various high-altitude seeing effects.
Conclusions: We conclude that statistical compensation of high-order modes is a viable method to reduce the loss of contrast occurring when a limited number of aberrations is explicitly compensated for with MFBD and JPDS processing. We show that good such compensation is possible with only 10 recorded frames. The main limitation of the method is that already MFBD and JPDS processing introduces high-order compensation that, if not taken into account, can lead to over-compensation. 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: 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: 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: 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: 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: What are 'Faculae'? Authors: Berger, T. E.; Title, A. M.; Tarbell, T.; Rouppe van der Voort, L.; Löfdahl, M. G.; Scharmer, G. B. Bibcode: 2007ASPC..369..103B Altcode: We present very high resolution filtergram and magnetogram observations of solar faculae taken at the Swedish 1-meter Solar Telescope (SST) on La Palma. Three datasets with average line-of-sight angles of 16, 34, and 53 degrees are analyzed. The average radial extent of faculae is at least 400~km. In addition we find that contrast versus magnetic flux density is nearly constant for faculae at a given disk position. These facts and the high resolution images and movies reveal that faculae are not the interiors of small flux tubes - they are granules seen through the transparency caused by groups of magnetic elements or micropores ``in front of'' the granules. Previous results which show a strong dependency of facular contrast on magnetic flux density were caused by bin-averaging of lower resolution data leading to a mixture of the signal from bright facular walls and the associated intergranular lanes and micropores. The findings are relevant to studies of total solar irradiance (TSI) that use facular contrast as a function of disk position and magnetic field in order to model the increase in TSI with increasing sunspot activity. Title: Recent High Resolution Observations and Interpretations of Sunspot Fine Structure Authors: Scharmer, G. B.; Langhans, K.; Kiselman, D.; Löfdahl, M. G. Bibcode: 2007ASPC..369...71S Altcode: We review analyses made of highly resolved filtergrams, magnetograms and Dopplergrams of sunspots, recorded with the Swedish 1-meter Solar Telescope (SSTSST) on La Palma. Dark cores in penumbral filaments are shown to be directly linked to peripheral umbral dots and to dark lanes in light bridges, suggesting similar or related underlying physics. The visibility of dark cores is found to depend strongly on the azimuth angle already for spots located at small heliocentric distances. It is shown that dark cores are clearly visible close to the center of the Ca II H line, formed approximately 150--200~km above the photosphere. We conclude that the τ = 1 layer of dark-cored filaments outlines a strongly warped surface, consistent with the finding that the magnetic field strength is strongly reduced in dark cores. We show that several properties of dark-cored filaments derived from SSTSST data are consistent with results of inversions of low-resolution Stokes spectra, but also find important discrepancies with the interpretation that penumbral filaments can be identified with flux tubes. Our data are consistent with the model proposed by Spruit & Scharmer (2006), explaining dark cores as signatures of field-free convection occurring just below the visible surface of the penumbra. We discuss recent simulations of light bridges and umbral dots, providing additional support to that model. Title: Contrast Analysis of Solar Faculae and Magnetic Bright Points Authors: Berger, T. E.; Rouppe van der Voort, L.; Löfdahl, M. Bibcode: 2007ApJ...661.1272B Altcode: The morphology and contrast of small-scale solar magnetic elements at four disk positions is analyzed. The data were obtained at the Swedish 1 m Solar Telescope (SST) over 3 yr (2003-2005). Two of the data sets have disk positions near disk center (average μ=cosθ>0.8) and show numerous ``magnetic bright points'' (MBPs), and two are sufficiently limbward to show prominent ``faculae'' (average μ<=0.6). The filtergrams are obtained in the 430.5 nm G band and 436.4 nm ``continuum'' bandpasses; the magnetograms are Fe I 630.25 nm Stokes V images taken with the Solar Optical Universal Polarimeter (SOUP) tunable filter. In all images we achieve nearly diffraction-limited resolution (~100 km in the G band). Analysis shows that MBPs and faculae are distinct radiative signatures of the magnetic field: MBPs have a constant or slightly decreasing contrast with increasing magnetogram signal, while facular contrast increases linearly with magnetogram signal. Faculae are much larger than MBPs, with an average radial width of 400 km. The observations support recent modeling showing that faculae are granules seen through the opacity reduction provided by magnetic elements (or groups thereof), while MBPs are caused by lateral radiation leakage scattering from deeper layers of the magnetic element. Title: Observations of dark-cored filaments in sunspot penumbrae Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G. Bibcode: 2007A&A...464..763L Altcode: Context: The recent discovery of dark-cored penumbral filaments suggests that we are resolving the building blocks of sunspot penumbrae. Their properties are largely unknown but provide important clues to understanding penumbral fine structure.
Aims: Our observations provide new constraints for the different scenarios put forward to explain the structure of sunspot penumbrae.
Methods: We present an analysis of dark-cored penumbral filaments, based on intensity filtergrams (G-band, continuum and Ca II H line wing), magnetograms and Dopplergrams, obtained at heliocentric distances between 15° and 55°.
Results: In general, the visibility of dark cores degrades with increasing heliocentric distance. Based on Ca II H wing images we conclude that this is due to a geometrical 3D-effect and not due to a simple formation height effect. Only in the center-side penumbra are dark-cored filaments visible at all observed heliocentric distances. We observe that dark-cored filaments frequently split in the umbra, forming a Y-shape that disappears after a few minutes, leaving a shortened filamentary structure and a bright dot in the umbra. The dark-cored filaments have life times ≥ 90 min. The dark cores are related to a much weaker and a more horizontal magnetic field than their lateral brightenings. Where the dark-cored filaments appear in the umbra, the magnetic field is inclined by 40° with respect to the solar surface normal for both the dark core and the bright edges. With increasing distance from the umbra, the magnetic field inclination in the dark cores increases rapidly within a few thousand km. Both the magnetic field strength and inclination in the lateral brightenings show very small variations with spot-center radial distance. The velocity field possesses a strong horizontal component within the dark cores. The absolute line-of-sight (LOS) velocity is larger within the dark cores than in their lateral brightenings. The Evershed flow apparently is present primarily in the dark cores. Title: A comparative study of the contrast of solar magnetic elements in CN and CH Authors: Zakharov, V.; Gandorfer, A.; Solanki, S. K.; Löfdahl, M. Bibcode: 2007A&A...461..695Z Altcode: No abstract at ADS Title: Solar image restoration Authors: Löfdahl, M. G.; van Noort, M. J.; Denker, C. Bibcode: 2007msfa.conf..119L Altcode: Image restoration is used to repair solar images degraded by the turbulence in Earth's atmosphere. Restoration algorithms are based on models of the optical system that produce the images from the solar source of radiation, through Earth's atmosphere and telescope/instrument optics, to the detectors recording the data. In this review, these model components are discussed in the context of two very different classes of image restoration methods, i.e., Speckle Imaging and Phase Diversity/Multi-Frame Blind Deconvolution, which have been successfully used during the last two decades. The strengths and weaknesses of these two approaches are discussed, as well as some variants and recent progress. Title: Solar Image Restoration by use of Multi-Object Multi-Frame Blind Deconvolution Authors: van Noort, M.; Rouppe van der Voort, L.; Löfdahl, M. Bibcode: 2006ASPC..354...55V Altcode: We present examples of the application of the image restoration method of Multi-Object Multi-Frame Blind Deconvolution to observations obtained with the Swedish one-meter Solar Telescope on La Palma. This restoration method is an extension of Joint Phase Diverse Speckle image restoration. Multiple realizations of multiple objects can now be restored jointly, facilitating near-perfect alignment between different objects. This greatly reduces false signals in the determination of derived quantities, such as magnetograms, Dopplergrams and G-band-continuum difference images. Title: Rapid Temporal Variability of Faculae: High-Resolution Observations and Modeling Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort, L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G. Bibcode: 2006ApJ...646.1405D Altcode: We present high-resolution G-band observations (obtained with the Swedish 1 m Solar Telescope) of the rapid temporal variability of faculae, which occurs on granular timescales. By combining these observations with magnetoconvection simulations of a plage region, we show that much of this variability is not intrinsic to the magnetic field concentrations that are associated with faculae, but rather a phenomenon associated with the normal evolution and splitting of granules. We also show examples of facular variability caused by changes in the magnetic field, with movies of dynamic behavior of the striations that dominate much of the facular appearance at 0.1" resolution. Examples of these dynamics include merging, splitting, rapid motion, apparent fluting, and possibly swaying. Title: A comparative study of the contrast of solar magnetic elements in CN and CH Authors: Zakharov, V.; Gandorfer, A.; Solanki, S. K.; Löfdahl, M. Bibcode: 2005A&A...437L..43Z Altcode: Photospheric bright points were investigated in three different wavelength bands using interference filters centered at 436.5 nm (continuum), 430.5 nm (Fraunhofer's G-band dominated by absorption due to CH), and 388.7 nm (absorption band of CN). Such bright points serve as proxies of small-scale solar magnetic elements. Near diffraction limited imaging was achieved by real-time frame selection and subsequent joint phase diverse speckle reconstruction. Comparison of the filtergrams of NOAA0670 taken in CH and CN shows that the contrast of bright points is on average 1.4 times

higher in CN than in G-band, which is in good quantitative agreement with the predictions of Berdyugina et al. (2003, A&A, 412, 513) and Rutten et al. (2001, ASP Conf. Ser., 236, 445). Title: Inclination of magnetic fields and flows in sunspot penumbrae Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Berger, T. E. Bibcode: 2005A&A...436.1087L Altcode: An observational study of the inclination of magnetic fields and flows in sunspot penumbrae at a spatial resolution of 0.2 arcsec is presented. The analysis is based on longitudinal magnetograms and Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma using the Lockheed Solar Optical Universal Polarimeter birefringent filter. Data from two sunspots observed at several heliocentric angles between 12 ° and 39 ° were analyzed. We find that the magnetic field at the level of the formation of the Fe i-line wing (630.25 nm) is in the form of coherent structures that extend radially over nearly the entire penumbra giving the impression of vertical sheet-like structures. The inclination of the field varies up to 45 ° over azimuthal distances close to the resolution limit of the magnetograms. Dark penumbral cores, and their extensions into the outer penumbra, are prominent features associated with the more horizontal component of the magnetic field. The inclination of this dark penumbral component - designated B - increases outwards from approximately 40 ° in the inner penumbra such that the field lines are nearly horizontal or even return to the solar surface already in the middle penumbra. The bright component of filaments - designated A - is associated with the more vertical component of the magnetic field and has an inclination with respect to the normal of about 35 ° in the inner penumbra, increasing to about 60 ° towards the outer boundary. The magnetogram signal is lower in the dark component B regions than in the bright component A regions of the penumbral filaments. The measured rapid azimuthal variation of the magnetogram signal is interpreted as being caused by combined fluctuations of inclination and magnetic field strength. The Dopplergrams show that the velocity field associated with penumbral component B is roughly aligned with the magnetic field while component A flows are more horizontal than the magnetic field. The observations give general support to fluted and uncombed models of the penumbra. The long-lived nature of the dark-cored filaments makes it difficult to interpret these as evidence for convective exchange of flux tubes. Our observations are in broad agreement with the two component model of Bellot Rubi et al. (2003), but do not rule out the embedded flux tube model of Solanki & Montavon (1993). Title: High resolution magnetogram measurements of solar faculae Authors: Berger, T. E.; Rouppe van der Voort, L.; Lofdahl, M. G. Bibcode: 2005AGUSMSP31A..02B Altcode: We present new images of magnetic elements near the solar limb ("faculae") along with magnetogram measurements and contrast profiles. Imaging magnetogram observations were made of AR 10377 at μ = cosθ = 0.6 on 06-June-2003 using the Swedish 1-m Solar Telescope; the data are the highest spatial resolution magnetogram measurements of faculae to date. Contrary to previous lower resolution measurements, we find no correlation between facular contrast and magnetic flux density at a given disk position. Increasing magnetic flux density in a region implies an increasing prevalence of micropores. Previous observations which lacked sufficient spatial resolution to discern dark micropore "floors" from bright facular walls find a strong non-linear dependence of facular contrast on magnetic flux density, with decreasing contrast beyond a certain flux density. We show instead that the observed contrast of bright facular walls is independent of magnetic flux density when properly segmented from dark micropores. The observations are useful for examining the detailed structure of faculae including the dark lanes found on the disk-center side of many faculae (explained by several recent 3D MHD numerical simulations). The average radial profile for 678 faculae segmented from the dataset is very nearly gaussian with a FWHM radial extent of 265 km and an extended tail on the limbward side, as predicted by current MHD simulations. Title: Solar Image Restoration By Use Of Multi-frame Blind De-convolution With Multiple Objects And Phase Diversity Authors: Van Noort, Michiel; Der Voort, Luc Rouppe Van; Löfdahl, Mats G. Bibcode: 2005SoPh..228..191V Altcode: 2005SoPh..228..191N An extension of Joint Phase Diverse Speckle image restoration is presented. Multiple realizations of multiple objects having known wavefront relations with each other can now be restored jointly. As the alignment of the imaging setup does not change, near-perfect alignment can be achieved between different objects, thus greatly reducing false signals in the determination of derived quantities, such as magnetograms, Dopplergrams, etc. The method was implemented in C++ as an image restoration server, to which worker clients can connect and disconnect randomly, so that a large number of CPUs can be used to speed up the restorations. We present a number of examples of applications of the restoration method to observations obtained with the Swedish 1-m Solar Telescope on La Palma. Title: Solar magnetic elements at 0.1 arcsec resolution. General appearance and magnetic structure Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.; Title, A.; Scharmer, G. Bibcode: 2004A&A...428..613B Altcode: New observations of solar magnetic elements in a remnant active region plage near disk center are presented. The observations were obtained at the recently commissioned Swedish 1-m Solar Telescope on La Palma. We examine a single 430.5 nm G-band filtergram that resolves ∼70 km (0.1 arcsec) structures and find new forms of magnetic structures in this particular region. A cotemporal Ca II H-line image is used to examine the low-chromosphere of network elements. A cotemporal Fe I 630.25 nm magnetogram that resolves structures as small as 120 km (0.18 arcsec) FWHM with a flux sensitivity of approximately 130 Mx cm-2 quantifies the magnetic structure of the region. A Ni I 676.8 nm Dopplergram establishes relative velocity patterns associated with the network features with an accuracy of about 300 m s-1. We find that magnetic flux in this region as seen in both the magnetogram and the G-band image is typically structured into larger, amorphous, ``ribbons'' which are not resolved into individual flux tubes. The measured magnetic flux density in the ribbon structures ranges from 300 to 1500 Mx cm-2, the higher values occurring at localized concentrations embedded within the ribbons. The Dopplergram indicates relative downflows associated with all magnetic elements with some indication that higher downflows occur adjacent to the peak magnetic flux location. The mean absolute magnetic flux density of the remnant plage network is about 130 Mx cm-2; in the lowest flux regions of the field-of-view, the mean absolute flux density is approximately 60 Mx cm-2. Within these quiet regions we do not find evidence of pervasive kilo-gauss strength magnetic elements as seen in recent high resolution internetwork studies. In general, the observations confirm recent 3-dimensional numerical simulations which show that the magnetic field in high-density regions such as plage is concentrated in complex structures that are not generally composed of discrete magnetic flux tubes.

Appendices are only available in electronic form at http://www.edpsciences.org Title: Observations of solar magnetic elements with 0.1" resolution Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Lofdahl, M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.; Title, A. M.; Scharmer, G. Bibcode: 2004AAS...204.2005B Altcode: 2004BAAS...36..686B New observations of solar magnetic elements in a remnant active region plage near disk center are presented. The observations were taken with the Swedish 1-meter Solar Telescope on La Palma. We examine a single 430.5 nm G-band filtergram that resolves ∼70 km (0.''1) structures and find new forms of magnetic structures in this particular region. A simultaneous Ca II H-line image is used to examine the low-chromosphere of network elements. A simultaneous Fe I 630.25 nm magnetogram that resolves structures as small as 120 km (0.''18) FWHM with a flux sensitivity of approximately 130 Mx cm-2 quantifies the magnetic structure of the region. A Ni I 676.8 nm Dopplergram establishes relative velocity patterns associated with the network features with an accuracy of about 300 m s-1. Magnetic flux in this region as seen in both the magnetogram and the G-band image is typically structured into larger, amorphous, ``ribbons'' with a wide range of flux density values, rather than isolated kilogauss flux tubes. We also present filtergrams and magnetograms of magnetic elements at the solar limb showing that solar faculae are resolved into bright granular walls that appear to project 350 to 500 km above the photosphere. Title: Innovative Information Technology for Space Weather Research Authors: Wang, H.; Qu, M.; Shih, F.; Denker, C.; Gerbessiotis, A.; Lofdahl, M.; Rees, D.; Keller, C. Bibcode: 2004AAS...204.5209W Altcode: 2004BAAS...36..755W Solar activity is closely related to the near earth environment -- summarized descriptively as space weather. Changes in space weather have adverse effect on many aspects of life and systems on earth and in space. Real-time, high-quality data and data processing would be a key element to forecast space weather promptly and accurately. Recently, we obtained a funding from US National Science Foundation to apply innovative information technology for space weather prediction.

(1) We use the technologies of image processing and pattern recognition, such as image morphology segmentation, Support Vector Machines (SVMs), and neural networks to detect and characterize three important solar activities in real-time: filament eruptions, flares, and emerging flux regions (EFRs). Combining the real time detection with the recent statistical study on the relationship among filament eruptions, flares, EFRs, coronal mass ejections (CMEs), and geomagnetic storms, we are establishing real time report of solar events and automatic forecasting of earth directed CMEs and subsequent geomagnetic storms.

(2) We combine state-of-art parallel computing techniques with phase diverse speckle imaging techniques, to yield near real-time diffraction limited images with a cadence of approximately 10 sec. We utilize the multiplicity of parallel paradigms to optimize the calculation of phase diverse speckle imaging to improve calculation speed. With such data, we can monitor flare producing active regions continuously and carry out targeted studies of the evolution and flows in flare producing active regions.

(3) We are developing Web based software tools to post our processed data, events and forecasting in real time, and to be integrated with current solar activity and space weather prediction Web pages at BBSO. This will also be a part of Virtual Solar Observatory (VSO) being developed by the solar physics community.

This research is supported by NSF ITR program. Title: Penumbral structure at 0.1 arcsec resolution. I. General appearance and power spectra Authors: Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Kiselman, D.; Scharmer, G. B. Bibcode: 2004A&A...414..717R Altcode: We analyse sunspot filtergrams of unprecedented quality obtained by \citet{scharmer02dark} with the Swedish 1-m Solar Telescope on La Palma. The observations comprise images in three different wavelength bands: 488, 436, and 430 nm (G-band). We find that there are still unresolved penumbral filaments which must have widths smaller than 80 km. The fine structuring along the filaments is limited. Penumbral grains have internal structure and look like they are split or crossed by narrow dark structures. We calculate intensity power spectra of the penumbra from images that are corrected for seeing using the Phase Diversity technique. The effects of high order aberrations that are not corrected for are estimated to be too low to be consistent with a flat power spectrum. The penumbral power spectra do not show any signs of bumps or peaks that could correspond to a preferred scale at 0\farcs35 for the width of penumbral filaments. We argue that the power spectrum is not a very reliable source of information concerning preferred scales. Title: Intensity Oscillations in the upper transition region above active region plage Authors: de Pontieu, B.; Erdelyi, R.; de Wijn, A.; Loefdahl, M. Bibcode: 2003AGUFMSH42B0540D Altcode: Although there are now many observations showing the presence of oscillations in the corona, almost no observational studies have focused on the bright upper transition region (TR) emission (so-called moss) above active region plage. Here we report on a wavelet analysis of observations (made with TRACE, the Transition Region and Coronal Explorer) of strong ( ∼ 5-15%) intensity oscillations in the upper TR footpoints of hot coronal loops. They show a range of periods from 200 to 600 seconds, typically persisting for 4 to 7 cycles. These oscillations are not associated with sunspots, as they usually occur at the periphery of plage regions. A majority of the upper TR oscillations are directly associated with upper chromospheric oscillations observed in Hα , i.e., periodic flows in spicular structures. The presence of such strong oscillations at low heights (of order 3,000 km) provides an ideal opportunity to study the propagation of oscillations from photosphere and chromosphere into the TR and corona, and improve our understanding of the magnetic connectivity in the chromosphere and TR. In addition, we use new high resolution observations of the photosphere and chromosphere, taken with the Swedish Solar Telescope, to shed light on the source of chromospheric mass flows such as spicules. Title: Observations of magnetoconvection in Sunspots with 100 km resolution Authors: Berger, T. E.; Löfdahl, M. G.; Scharmer, G.; Title, A. M. Bibcode: 2003SPD....34.1108B Altcode: 2003BAAS...35..828B We present new observations from the Swedish 1-meter Solar Telescope (SST) on La Palma with ∼0.1 arcsecond ( ∼100 km) resolution: the highest resolution yet achieved in solar observations. We focus on sunspot and active region magnetoconvective phenomena using G-band 4305 Å, 4877 Å continuum, 7507 Å TiO bandhead, and Ca II 3968 Å H-line filtergram movies. The G-band data are post-processed using Joint Phase Diverse Speckle wavefront restoration to create a full diffraction limited time series. Sunspot light-bridges are shown to have dark lanes less than 300 km in width that are coherent along the entire length of the bridge. Similarly, we find elongated dark ``canals'' in plage regions, particularly near pores, that appear to be highly modified intergranular downflow lanes. The canals are less than 200 km in width and are much more coherent than intergranular lanes in non-magnetic regions, often retaining their basic structure for more than one granular turn-over time. Both the light-bridge central lane and the canals appear to be the result of highly constrained flow structure in strong magnetic field regions -- an aspect of solar magnetoconvection that has not previously been observed. This reseach was supported by funding from the Royal Swedish Academy of Sciences, a SOHO Guest Investigator subcontract to California State University Northridge, and the NASA TRACE contract NAS5-38099 at Lockheed Martin. Title: Adaptive optics system for the new Swedish solar telescope Authors: Scharmer, Goran B.; Dettori, Peter M.; Lofdahl, Mats G.; Shand, Mark Bibcode: 2003SPIE.4853..370S Altcode: The 1-meter Swedish solar telescope is a new solar telescope that was put in operation on the island of La Palma in the Canary Islands at the end of May 2002. The goal of this telescope is to reach its diffraction limited resolution of 0.1 arcsec in blue light. This has already been achieved by use of a low-order adaptive optics (AO)system. This paper describes the AO system initially developed for the former 50-cm Swedish Vacuum Solar Telescope (SVST) and further improved for the new telescope. Both systems use a combination of bimorph modal mirrors and Shack-Hartmann wavefront sensors. Unique to these systems are that they rely on a single workstation or a PC to do all the computations required to extract and pre-process the images, measure their positions using cross correlation techniques and for controlling the deformable mirror. This is in the present system possible by using the PERR instruction available on Compaq's Alpha architecture and in the new system using the PSADDBW instruction, available on Pentium 4 and Athlon processors. We describe both these systems with an emphasis on the performance, the ease of support and upgrades of performance. We also describe the optimization of the electrode geometry for the new 37-electrode bimorph mirror, supplied by AOPTIX Technologies, Inc., for controlling Karhunen--Loeve modes. Expected performance, based on closed-loop simulations, is discussed. Title: Phase diverse speckle inversion applied to data from the Swedish 1-meter solar telescope Authors: Lofdahl, Mats G.; Scharmer, Goran B. Bibcode: 2003SPIE.4853..567L Altcode: We report on the use of a new joint phase diverse speckle code, an implementation of a method where a single object and individual phases are estimated from several pairs of phase diverse data. The code was used on 430.5 nm G-band data collected with the newly installed Swedish 1-meter solar telescope in La Palma, equipped with a low-order adaptive optics system. We describe the algorithm briefly, show wavefront statistics and object estimates from the processing and discuss the results. We demonstrate a resolution of 0.12 arc seconds for a time sequence and a large field of view, which is a break-through for ground based solar telescopes. Title: First Results from the Swedish 1-m Solar Telescope Authors: Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Rouppe van der Voort, L. H. M. Bibcode: 2003ASPC..307....3S Altcode: No abstract at ADS Title: Multi-frame blind deconvolution with linear equality constraints Authors: Löfdahl, Mats G. Bibcode: 2002SPIE.4792..146L Altcode: 2002physics...9004L The Phase Diverse Speckle (PDS) problem is formulated mathematically as Multi Frame Blind Deconvolution (MFBD) together with a set of Linear Equality Constraints (LECs) on the wavefront expansion parameters. This MFBD-LEC formulation is quite general and, in addition to PDS, it allows the same code to handle a variety of different data collection schemes specified as data, the LECs, rather than in the code. It also relieves us from having to derive new expressions for the gradient of the wavefront parameter vector for each type of data set. The idea is first presented with a simple formulation that accommodates Phase Diversity, Phase Diverse Speckle, and Shack-Hartmann wavefront sensing. Then various generalizations are discussed, that allows many other types of data sets to be handled. Title: Dark cores in sunspot penumbral filaments Authors: Scharmer, Göran B.; Gudiksen, Boris V.; Kiselman, Dan; Löfdahl, Mats G.; Rouppe van der Voort, Luc H. M. Bibcode: 2002Natur.420..151S Altcode: Sunspot umbrae-the dark central regions of the spots-are surrounded by brighter filamentary penumbrae, the existence of which remains largely inexplicable. The penumbral filaments contain magnetic fields with varying inclinations and are associated with flowing gas, but discriminating between theoretical models has been difficult because the structure of the filaments has not hitherto been resolved. Here we report observations of penumbral filaments that reveal dark cores inside them. We cannot determine the nature of these dark cores, but their very existence provides a crucial test for any model of penumbrae. Our images also reveal other very small structures, in line with the view that many of the fundamental physical processes in the solar photosphere occur on scales smaller than 100km. Title: Observation and Modelling of Micropore Formation in Active Network Regions Authors: Berger, T. E.; Löfdahl, M. G.; Bercik, D. J. Bibcode: 2002AAS...200.9101B Altcode: 2002BAAS...34Q.953B We present phase-diversity corrected G-band 4305 Å and 4364 Å continuum image time series showing the formation of a micropore in a small active region near disk center. The data were acquired at the Swedish Vacuum Solar Telescope on La Palma in June of 1997 and post-processed using the Phase Diverse Speckle (PDS) algorithm to produce diffraction limited images throughout the majority of both time series. The micropore dataset comprises a 29x29 Mm field of view and spans 5.1 hours with a 38 second cadence. The micropore forms in a strong sink area that can be seen to ``collect" many G-band bright points over the first 2 hours of the observation. During this time there is an occasional darkening at the sink point that may be the first unstable phase of the micropore formation. Once a stable dark pore forms in the flowfield, it grows to a maximum diameter of 1.2 Mm in approximately 1.9 hours. The pore persists for another 35 minutes before apparently being broken up by the intergranular flowfield. The total ``lifetime" of the stable pore phase is 2.5 hours. A separate nearby micropore of 1.5 Mm maximum diameter exists for the entire 5.2 hour data span. We show G-band and continuum movies of the micropore formation, correlation tracking flowfield analyses, G-band bright point tracking results, and area versus time plots for the micropore formation lifetime. The observational data are compared with fully compressible 3D MHD numerical simulations which show the development of a similar micropore structure within the computational domain. This research was supported by NASA SR&T grant NASW-98008, The Royal Swedish Academy of Sciences, NSF and NASA funding at Michigan State University, and Lockheed Martin IRAD funding. Title: Two dual-wavelength sequences of high-resolution solar photospheric images captured over several hours and restored by use of phase diversity Authors: Löfdahl, M. G.; Berger, T. E.; Seldin, J. H. Bibcode: 2001A&A...377.1128L Altcode: The collection, seeing compensation, and temporal filtering of two high-resolution time-sequences of solar photospheric images are described. 44arcsecx80 arcsec image sequences of co-spatial and co-temporal 430.5 nm G band and 436.4 nm continuum filtergrams, were obtained with the 47.5 cm Swedish Vacuum Solar Telescope. One data set covers 5 hours of photospheric evolution; the other covers 8 hours. The field-of-view contains both an enhanced network region, a large pore, and in the longer sequence, a pore forming event. The mean time between frames is <40 s. With a few exceptions, the G band frames are partitioned phase-diverse speckle (PPDS) restorations of three realizations of the atmospheric turbulence acquired rapidly in sequence. Due to strict simultaneity and closely spaced wavelengths, the G band wavefronts, compensated for fixed aberration differences, could also be used for deconvolving the corresponding continuum data. For some of the data, collected during periods of particularly bad seeing, restorations made with a related method, joint phase-diverse speckle, were substituted for the PPDS restorations. Title: Workstation-based solar/stellar adaptive optics system Authors: Scharmer, Goran B.; Shand, Mark; Lofdahl, Mats G.; Dettori, Peter M.; Wei, Wang Bibcode: 2000SPIE.4007..239S Altcode: The microprocessors used in off-the-shelf workstations double in performance every eighteen months. The Swedish Vacuum Solar Tower (SVST) uses off-the-shelf workstations for all aspects of its on-line telescope control and data acquisition. Since 1995 workstation performance has been adequate for a correlation tracker of solar granulation controlling a tip- tilt corrector. In 2000 workstation performance permits the construction of a 20 - 50 subimage Shack-Hartmann based low- latency adaptive optics system. It is argued that workstations provide a cost-effective, upgradable, low-risk and flexible means of construction of stellar and solar adaptive optics systems. We give an overview of the adaptive optics system installed at the SVST in May 1999. The system uses a bimorph modal mirror with 19 electrodes from Laplacian Optics. For use with extended targets, such as solar fine structure, cross- correlations with 16 X 16-pixel sub-images are used. For use with point sources, a centroiding algorithm is implemented. The work station used is capable of completing all processing required by the adaptive optics system in 0.5 ms (cross-correlations) or 0.3 ms (centroiding), with potential for significant performance improvements. Title: Resolving piston ambiguities when phasing a segmented mirror Authors: Lofdahl, Mats G.; Eriksson, Henrik Bibcode: 2000SPIE.4013..774L Altcode: Wavefront sensing in monochromatic light is insensitive to segment piston errors that are a whole number of waves. If the wavefront sensing is performed in several wavelengths, this ambiguity can be resolved. We give an algorithm for finding the correct phase, given multiple measurements in different wavelengths. Using this algorithm, the capture range of a wavefront sensor can be extended from on the order of +/- (lambda) /2 in piston to several waves. This relaxes the demands on an initial, coarse alignment method. The extended capture range depends on the selection of wavelengths available for phase measurements and the expected accuracy of the wavefront sensing method used. Title: Predictor approach to closed-loop phase-diversity wavefront sensing Authors: Lofdahl, Mats G.; Scharmer, Goran B. Bibcode: 2000SPIE.4013..737L Altcode: We present a novel and fast method for utilizing wavefront information in closed-loop phase-diverse image data. We form a 2D object-independent error function using the images at different focus positions together with OTFs of the diffraction limited system. Each coefficient in an expansion of the wavefront is estimated quickly and independently by calculating the inner produce of a corresponding predictor function and the error function. This operation is easy to parallelize. The main computational burden is in pre- processing, when the predictors are formed. This makes this method fast and therefore attractive for closed loop operation. Calculating the predictors involves error function derivatives with respect to the wavefront parameters, statistics of the parameters, noise levels and other known characteristics of the optical system. The predictors are optimized so that the RMS error in the wavefront parameters is minimized rather than consistency between estimated quantities with image data. We present simulation results that are relevant to the phasing of segmented mirrors in a space telescope, such as the NGST. Title: Calibration of a Deformable Mirror and Strehl Ratio Measurements by Use of Phase Diversity Authors: Löfdahl, Mats G.; Scharmer, Göran B.; Wei, Wang Bibcode: 2000ApOpt..39...94L Altcode: No abstract at ADS Title: Orthogonalization of Basis Functions for Diagonalized Wavefront Sensing Authors: Löfdahl, M. Bibcode: 1999ASPC..183..320L Altcode: 1999hrsp.conf..320L No abstract at ADS Title: Experimental results from a multiple telescope imaging array. Authors: Kendrick, R. L.; Bell, R.; Benson, L.; Cuneo, P.; Duncan, A. L.; Holmes, B.; Lofdahl, M.; Mitchell, K.; Reardon, B.; Sigler, R.; Stone, R.; Stubbs, D.; Zarifis, V. Bibcode: 1999aero....5..415K Altcode: A nine telescope imaging phased array is being demonstrated at the Lockheed-Martin Advanced Technology Center in Palo Alto. The array consists of nine afocal telescopes arranged in a y-formation that are combined to a common focus in a Fizeau interferometer configuration. The authors have demonstrated diffraction limited performance over a 150 micro-radian field of view with broadband (500 to 800 nm) illumination. Title: The Multi Aperture Imaging Array Authors: Zarifis, V.; Bell, R. M., Jr.; Benson, L. R.; Cuneo, P. J.; Duncan, A. L.; Herman, B. J.; Holmes, B.; Sigler, R. D.; Stone, R. E.; Stubbs, D. M.; Kendrick, R. L.; Paxman, R. G.; Seldin, J. H.; Löfdahl, M. G. Bibcode: 1999ASPC..194..278Z Altcode: 1999wfoi.conf..278Z A multi-aperture telescope imaging phased array is being demonstrated at the Lockheed Martin Advanced Technology Center in Palo Alto. The sparse array consists of nine afocal telescopes that are combined to a common focus in a Fizeau interferometer configuration. We have demonstrated diffraction limited performance over a 150 micro-radian field of view with broad band (550 to 800 nm) illumination. The relatively high bandwidth closed loop phasing is achieved using phase diversity wavefront sensing techniques. Applications using a filled array for astronomical observations will also be discussed. Title: Dynamics of Magnetic Flux Elements in the Solar Photosphere Authors: van Ballegooijen, A. A.; Nisenson, P.; Noyes, R. W.; Löfdahl, M. G.; Stein, R. F.; Nordlund, Å.; Krishnakumar, V. Bibcode: 1998ApJ...509..435V Altcode: 1998astro.ph..2359V The interaction of magnetic fields and convection is investigated in the context of the coronal heating problem. We study the motions of photospheric magnetic elements using a time series of high-resolution G-band and continuum filtergrams obtained at the Swedish Vacuum Solar Telescope at La Palma. The G-band images show bright points arranged in linear structures (``filigree'') located in the lanes between neighboring granule cells. We measure the motions of these bright points using an object tracking technique, and we determine the autocorrelation function describing the temporal variation of the bright point velocity. The correlation time of the velocity is about 100 s. To understand the processes that determine the spatial distribution of the bright points, we perform simulations of horizontal motions of magnetic flux elements in response to solar granulation flows. Models of the granulation flow are derived from the observed granulation intensity images using a simple two-dimensional model that includes both inertia and horizontal temperature gradients; the magnetic flux elements are assumed to be passively advected by this granulation flow. The results suggest that this passive advection model is in reasonable agreement with the observations, indicating that on a timescale of 1 hr the flux tubes are not strongly affected by their anchoring at large depth. Finally, we use potential-field modeling to extrapolate the magnetic and velocity fields to larger height. We find that the velocity in the chromosphere can be locally enhanced at the separatrix surfaces between neighboring flux tubes. The predicted velocities are several km s-1, significantly larger than those of the photospheric flux tubes. The implications of these results for coronal heating are discussed. Title: Fine Structures of Magnetic Field in Solar Quiet Region Authors: Zhang, H.; Scharmer, G.; Lofdahl, M.; Yi, Z. Bibcode: 1998SoPh..183..283Z Altcode: In this paper, we present a time series of Fe i λ5250.2 Å photospheric filtergrams and corresponding magnetograms in a quiet region. The relationship between fine structures of granulation and magnetic fields is analyzed. It is found that although most bright filigree features in photospheric filtergrams are related to corresponding magnetic features, they are generally not cospatial. It is also found that some bright features and their corresponding photospheric magnetic fields show fast changes within several minutes. Title: Measurements of Solar Magnetic Element Dispersal Authors: Berger, Thomas E.; Löfdahl, Mats G.; Shine, Richard A.; Title, Alan M. Bibcode: 1998ApJ...506..439B Altcode: The dispersal of magnetic elements in the solar photospheric flow field is studied by tracking individual ``magnetic bright points'' (MBPs) identified in a G-band 4305 Å filtergram time series obtained at the 50 cm Swedish Vacuum Solar Telescope on La Palma, Spain. The time series spans approximately 70 minutes with a field of view of 29" × 29" near disk center. All images in the time series are restored to near the telescope diffraction limit (~0.2" in the G band) using partitioned phase diverse speckle techniques. Regions of enhanced magnetic network and quiet Sun are examined. In the network region, automated tracking of individual MBPs reveals approximately Gaussian diffusion, with indications for slightly ``superdiffusive'' dispersal. The inferred Gaussian diffusion coefficient is 60.4 +/- 10.9 km2 s-1. In the quiet-Sun region, local correlation tracking velocity measurements show the dispersal of artificial tracers to be non-Gaussian over most of our data set with indications of an asymptotic approach to a 285 km2 s-1 Gaussian diffusion. Title: Fast phase diversity wavefront sensing for mirror control Authors: Lofdahl, Mats G.; Duncan, Alan L.; Scharmer, Goran B. Bibcode: 1998SPIE.3353..952L Altcode: We show with simulation experiments that closed-loop phase- diversity can be used without numerical guard-bands for wavefront sensing of low-order wavefronts from extended objects using broad-band filters. This may allow real-time correction at high bandwidth for certain applications. We also present a proper maximum likelihood treatment of Shack- Hartmann data, which includes an imaging model to extract curvature information from the lenslet images. We demonstrate by simple simulations that this approach should allow higher-order wavefront information to be extracted than with traditional Shack-Hartmann wavefront sensing for a given number of lenslets. Title: Phase diversity experiment to measure piston misalignment on the segmented primary mirror of the Keck II Telescope Authors: Lofdahl, Mats G.; Kendrick, Richard L.; Harwit, Alex; Mitchell, Keith E.; Duncan, Alan L.; Seldin, John H.; Paxman, Richard G.; Acton, Daniel S. Bibcode: 1998SPIE.3356.1190L Altcode: We are developing a technique to measure segment misalignment of large telescopes based on wavefront estimation using phase-diverse images. We report the current results of an experiment to measure piston errors on the Keck II primary segmented mirror, through atmospheric turbulence, using phase-diverse phase retrieval. The segment piston errors are separated from the random turbulence by averaging phase estimates from many frames. Phase estimates from real data collected with segments intentionally moved in piston reproduce the observed speckle patterns well. However, average phase maps do not reveal the segment piston errors. Simulations show that the observed data were collected in a regime of turbulence where the current algorithm often fails, but would be expected to work very well when the adaptive optics system is operating. There is reason to believe that we can eventually make the algorithm work with these or similar data if apparent mismatches between the data and our current imaging model are removed. Title: Preparation of a Dual Wavelength Sequence of High-Resolution Solar Photospheric Images Using Phase Diversity Authors: Löfdahl, Mats G.; E. Berger, Thomas; Shine, Richard S.; Title, Alan M. Bibcode: 1998ApJ...495..965L Altcode: The collection, seeing compensation, and temporal filtering of a high-resolution time-sequence of solar photospheric images is described. A 70 minute time series of cospatial and cotemporal G band 4305 Å and wideband 4686 Å filtergrams was obtained with the 50 cm Swedish Vacuum Solar Telescope on the island of La Palma, Spain. The 29" × 70" field-of-view near disk center contains both an enhanced network region and an (apparently) nonmagnetic ``quiet'' region of granulation. The mean time between frames is 23.5 s. Each frame is created with partitioned phase-diverse speckle restoration of three realizations of the atmospheric turbulence acquired rapidly in sequence. The result is high-resolution movies of the solar photosphere, good enough to allow detection, tracking, and analysis of ~0.2" bright points. This analysis is the subject of a companion paper. Title: Measurements of Solar Magnetic Element Motion from High-Resolution Filtergrams Authors: Berger, Thomas E.; Löfdahl, Mats G.; Shine, Richard S.; Title, Alan M. Bibcode: 1998ApJ...495..973B Altcode: Solar photospheric flowfield properties on sub-0.5" scales are measured using local correlation tracking (LCT) and object tracking of magnetic bright points (MBPs: photospheric bright points associated with magnetic elements). The dataset is a 70 minute time series of cospatial and cotemporal G-band 4305 Å and wideband 4686 Å filtergrams obtained with the 50 cm Swedish Vacuum Solar Telescope on the island of La Palma, Spain. We examine a 29" × 70" field of view (FOV) near disk center and compare a 29" × 29" magnetic network subfield and a 27" × 27" apparently nonmagnetic ``quiet-Sun'' subfield. The mean time between frames is 23.75 s. Each frame is created by partitioned phase-diverse speckle restoration of three image pairs acquired rapidly in sequence. Angular resolution is ~0.4" or less in all frames of the restored data set.

From LCT on a 0.4" grid with 0.83" FWHM apodization, we find the flow speeds to be Rayleigh distributed with a mode of 550 m s-1 and an average of 950 m s-1 in the network FOV; in the quiet FOV the modal speed is 700 m s-1 with a mean of 1100 m s-1. Within the network FOV, a ``magnetic region'' defined by the loci of tracked MBPs exhibit even greater alteration: rms contrast of the region is 8% higher in the G band compared to areas outside, LCT speeds are reduced by a factor of 1.6, and the convective flow structures are smaller and much more chaotic. Mesogranulation is entirely absent in the magnetic region. The modal and mean speeds of 534 tracked MBPs are 300 m s-1 and 1470 m s-1, respectively. MBPs split and merge with mean times of 320 and 404 s respectively. The mean lifetime of MBPs in the data set is 9.33 minutes although approximately 5% of the objects can be tracked for the entire 70 minute duration of the time series. Title: Phase-Diversity Restoration of two Simultaneous 70-minute Photospheric Sequences. Authors: Lofdahl, M. G.; Berger, T. E.; Shine, R. A.; Title, A. M. Bibcode: 1997SPD....28.0218L Altcode: 1997BAAS...29..896L Seeing effects have been corrected in two cospatial and cotemporal 70-minute sequences of images collected in the G-band 4305 Angstroms and wideband 4686 Angstroms. The data were obtained with the 50 cm Swedish Vacuum Solar Telescope on the island of La Palma, Spain. The 29arcsecx 70arcsec field-of-view (FOV) near disk center contains both an enhanced network region and an (apparently) non-magnetic ``quiet'' region of granulation. The mean time between restored frames is 23.5 s. Each of the 180 images is created with Phase-Diverse Speckle (PDS) imaging, using two different focus positions sampled at the best three snapshots of the atmospheric turbulence (seeing) during a 20-second selection window. Wavefronts are estimated for each focused--defocused image pair and a restored frame is produced from all six images. The average resolution in the restored sequence is about 0farcs4 (corresponding to spatial frequencies up to half the diffraction limit of the telescope), which is good enough to allow detection of ~ 0farcs2 bright points. The data is used for statistical measurements of magnetic element speed, interaction frequency, and lifetime (see accompanying poster by T. E. Berger et al). We show destretched and space-time filtered movies of both the G-band and continuum images, as well as raw data to demonstrate the effect of the restoration process. This work was supported by NASA contracts NAS5-30386 at Stanford and NAS8-39747 and Independent Research Funds at Lockheed-Martin. MGL was supported by the Swedish Science Research Council. Title: Measurements of Magnetic Element Dynamics in the Network Authors: Berger, T. E.; Lofdahl, M. G.; Shine, R. A.; Title, A. M. Bibcode: 1997SPD....28.0219B Altcode: 1997BAAS...29..896B Statistical measurements of magnetic element speed, interaction frequency, and lifetime in an enhanced network region at disk center are presented. The primary dataset is a 70 min time series of G-band 4305 Angstroms filtergrams taken at the 50 cm Swedish Vacuum Solar Telescope (SVST) on the island of La Palma, Spain. A second time series of 4686 Angstroms wide-band continuum filtergrams, cospatial and simultaneous to within several milliseconds with the G-band images, is also studied. The field-of-view is near Sun center and includes a region of enhanced network activity with many G-band bright points. Both time series are corrected for seeing to very near the telescope diffraction limit by the technique of Phase Diverse Speckle (PDS) restoration (see accompanying poster by M. G. Lofdahl et al). We show destretched and space-time filtered movies of both the G-band and continuum images in two fields-of-view: a region of quiet granulation and the region of enhanced network activity. Within the network, local correlation tracking measurements on a 0\farcs4 grid show the RMS speed to be 778 m s(-1) ; outside the network the RMS speed is 1168 m s(-1) . Corkflow simulations show that normal convective flow patterns (granulation and mesogranulation) are absent in the network. Magnetic elements move with modal and mean speeds of 100 m s(-1) and 815 m s(-1) , respectively. The mean interaction frequency (time between either merging or splitting) is 220 s. The mean lifetime of tracked magnetic elements in the network is 9.33 min although 5% of the elements are tracked for the entire 70 min of the times series. This work was supported by NASA contracts NAS5-30386 at Stanford and NAS8-39747 and Independent Research Funds at Lockheed-Martin. MGL was supported by the Swedish Science Research Council. Title: Evaluation of Phase-Diversity Techniques for Solar-Image Restoration Authors: Paxman, Richard G.; Seldin, John H.; Loefdahl, Mats G.; Scharmer, Goeran B.; Keller, Christoph U. Bibcode: 1996ApJ...466.1087P Altcode: Phase-diversity techniques provide a novel observational method for overcoming the effects of turbulence and instrument-induced aberrations in ground-based astronomy. Two implementations of phase-diversity techniques that differ with regard to noise model, estimator, optimization algorithm, method of regularization, and treatment of edge effects are described. Reconstructions of solar granulation derived by applying these two implementations to common data sets are shown to yield nearly identical images. For both implementations, reconstructions from phase-diverse speckle data (involving multiple realizations of turbulence) are shown to be superior to those derived from conventional phase-diversity data (involving a single realization). Phase-diverse speckle reconstructions are shown to achieve near diffraction-limited resolution and are validated by internal and external consistency tests, including a comparison with a reconstruction using a well-accepted speckle-imaging method. Title: Phase diversity wavefront sensing and image restoration applied to high-resolution solar observations Authors: Löfdahl, M. G. Bibcode: 1996PhDT........94L Altcode: An implementation of a phase-diverse speckle imaging (PDS) technique for reducing the effects of aberrations in solar images is described. Such aberrations usually occur in the Earth's atmosphere and in telescopes. PDS is a post-processing technique for measuring such aberrations and for deblurring the images.

The code has been extensively tested. Realistic simulations indicate that the systematic errors are small. The wavefront and object estimates calculated from real solar data, collected with the Swedish Vacuum Solar Telescope (SVST), are spatially and temporally consistent with expectations for anisoplanatism and the assumption of no evolution of the solar features on the time scale of a few seconds. Implementation invariance is demonstrated by comparison of the output with that of a separately developed implementation. External reference for the wavefront determination is provided by comparison with theoretical predictions of temporal variation of the telescopic aberrations at the SVST. High-quality image restorations can be made with much less data than is needed by the more established method of speckle interferometry.

The usefulness of the technique for astronomical purposes is demonstrated by the successful restoration and analysis of a 29"x29" 70-minute time sequence of solar granulation and bright points. The resolution in the restored data is sufficient to allow the evolution and motion of bright points to be followed in detail. Title: Wavefront sensing and image restoration from focused and defocused solar images. Authors: Löfdahl, M. G.; Scharmer, G. B. Bibcode: 1994A&AS..107..243L Altcode: We have implemented a least-squares technique for recovering phase information from simultaneously recorded focused and defocused images. The inversions are made from small subfields in order to deal with anisoplanatic image formation, such as occurring through the Earth's atmosphere. Parameters corresponding to the alignment of the focused-defocused images are determined simultaneously with aberration parameters. Simulations show that the method can recover wavefronts of up to 1/2 wave rms and that 15-21 Zernike coefficients can be obtained from 3"x3"-5"x5" solar granulation images using a 50 cm telescope and with noise levels of 0.4% of the average intensity. In general, the accuracy of the restored images is better than expected from the number of Zernike polynomials used to represent the wave front. We have applied the method to sequences of 100 8-bit solar granulation images obtained with the Swedish Vacuum Solar Telescope (SVST) in La Palma using subfields of 3"x3"-5"x5". These data enable a number of consistency tests, all of which demonstrate that the technique works on real data. Using averaged images obtained from each sequence we find that derived alignment parameters are consistent to within 0.02" and that wavefronts derived from different subfields and different sequences recorded close in time are virtually identical. The wavefronts derived from averaged images are also virtually identical to the average of wavefronts derived from individual images. These measurements of telescope aberrations suggest that astigmatism and coma are the major aberrations. These aberrations vary with time in a way which is consistent with a major contribution from the telescope objective and/or the first folding mirror of the alt-az tower telescope. Wavefronts derived from individual images show 50-90% correlation between Zernike coefficients 4-14 derived from nearby but independent subfields. Mosaics of 13 x 11 independently derived wavefronts from single images show smooth variations across a 12"x10" field-of-view. These results are consistent with the impression that degradation of image quality is more or less uniform across the image. Restored images in a sequence show a high degree of consistency and much more fine structure than the corresponding observed images, but occasional fringe-like artifacts can be seen. Using the results of two inversions to restore the scene removes such artifacts. We conclude that this technique provides adequate wavefront information on telescopic and atmospheric wavefront aberrations and substantial improvements in image quality. The ease of implementation as well as its tolerance to experimental errors and low cost makes it an excellent complement to or even substitute for adaptive optics for many applications. The technique is particularly well-suited for solar telescopes, where wavefront sensing over a large field-of-view is important. Title: Application of phase-diversity to solar images Authors: Lofdahl, Mats G.; Scharmer, Goran B. Bibcode: 1994SPIE.2302..254L Altcode: We have implemented a least-squares technique for recovering phase information and alignment parameters from simultaneously obtained focused and defocused solar images. Small subfields are used, in order to deal with anisoplanatism. The method is applied to sequences of 100 8-bit solar granulation images. These data enable a number of consistency tests, all of which demonstrate that the technique works. Alignment parameters derived from averaged images in a sequence are highly consistent and wavefronts derived from different subfields and different sequences recorded close in time are virtually identical. The wavefronts derived from averaged images are also virtually identical to the average of wavefronts derived from individual images. These aberrations vary with time in a way which is consistent with a major contribution from the moving elements of the alt-az tower telescope. Independently derived wavefronts from single images show high correlation between neighboring subfields and smooth variations across large fields-of-view, consistent with the impression that the image quality is more or less uniform across the image. Restored images in a sequence show a high degree of consistency and much more fine structure than the corresponding observed images. Title: Phase-Diversity Restoration of Solar Images Authors: Löfdahl, Mats G.; Scharmer, Göran B. Bibcode: 1993rtpf.conf...89L Altcode: We have implemented a least-squares linearization technique for recovering phase information from simultaneously recorded focused and defocused solar images. The inversion is made from small subfields, in order to deal with anisoplanatic image formation, such as occuring through the earth's atmosphere. Simulations show that the method can recover wavefronts of approximately 1/4 wave rms and that noise levels corresponding to 8-bit images allow better reconstruction of the images than of the derived wavefronts. We find that cross-correlation techniques cannot be used to align the focused and defocused images because of cross-talk with coma and similar asymmetric aberrations. Therefore we simultaneously determine alignment and aberration parameters. We have applied the method to 8-bit solar granulation images obtained with the Swedish Vacuum Solar Telescope in La Palma using subfields of 3".1 x 3".1. The least-squares fits indicate wavefronts of approximately 0.12-0.16 waves rms. Derived alignment parameters from different subfields and different images are consistent, indicating that the wavefront is well determined by the data. Wavefronts derived from different sub fields of a single image in general show similar aberrations whereas derived wavefronts from different images only show occasional similarities, indicating a significant wavefront aberration from the lower atmosphere. The reconstructed granulation images show enhanced fine structure and an increase of the rms contrast by ~2% as compared to the observed focused image, but the rms contrasts of consequtive images indicate small errors in the derived rms wavefronts. Title: The ROYACS User Library. Authors: Löfdahl, M. Bibcode: 1992lest.rept...65L Altcode: The main purpose with the ROYACS User Library is to provide ANA users at the Stockholm Observatory with suitable subprograms for data reduction of solar images. Other useful routines have also been included in order to improve the programming environment. Title: Swedish solar telescope: Short summary of instrumentation and observation techniques Authors: Scharmer, Goran; Lofdahl, Mats Bibcode: 1991AdSpR..11e.129S Altcode: 1991AdSpR..11..129S A short summary of the design concepts of the Swedish Solar Telescope at La Palma, is given along with the most important parts of the instrumentation and observing techniques. Our experiences from using high-speed read-out CCDs for solar observations are also discussed. The advantages of this data aquisition system are that it allows real-time frame selection for achieving high spatial resolution, that several cameras can be slaved by one seeing monitor, and that bursts of digital images can be recorded for full spatial coverage of small parts of active regions.