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Diffraction-limited Time Series of the Solar Atmosphere (1Sep95) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 43, September 1995) Many processes in the solar atmosphere have typical spatial scales of much less than one second of arc. If the processes are related to magnetic fields, the relevant scales are even smaller. It is therefore of great interest to reach the diffraction limit of the existing, relatively small solar telescopes. Noticeable evolution of solar features occurs on time-scales of less than a minute, if spatial resolution of better than one second of arc is reached. Speckle imaging has been used to obtain images at the diffraction limit of solar telescopes by taking about 100 frames for a single reconstruction. Time series with a temporal resolution of about 30s required very high data rates and produced an enormous amount of data. A reconstruction method that requires less data is therefore highly desirable. Richard G. Paxman, John H. Seldin (both ERIM, Michigan), Guenther Elste (Univ. of Michigan), and Christoph U. Keller (NSO Tucson) used the new technique of phase-diverse speckle imaging to obtain diffraction-limited time series of the solar atmosphere with the 76 cm Sacramento Peak Vacuum Tower Telescope. They used three CCD cameras of the Zrich Imaging Stokes Polarimeter I in parallel at a rate of seven frames per second per camera. One camera was in focus, another one out of focus, and a third one was behind a narrow-band tunable filter. The sharpness of each acquired triplet of images was analyzed in real time, and only the three sharpest triplets out of 50 triplets were stored for later use. [Photo not included] Reconstructions of a plage region with time runing from left to right, top to bottom. The time between images is about 50 s. Tick marks have a 1" spacing. The data were collected under moderate seeing conditions of about 1". Phase-diverse speckle imaging, developed by Paxman and Seldin over the last few years, was used to remove the aberrations of the atmosphere and the telescope by using the simultaneous in-focus and out-of-focus images in a 5 nm passband around 656 nm. The point-spread function determined in this relatively broad channel can then be used to restore the 0.05 nm bandpass images obtained behind the Universal Birefringent Filter. The figure shows a reconstructed time series in the 5 nm passband of a field slightly less than 4" by 4" in size. For this time sequence, a set of 300 image triplets was collected over a span of 13.5 minutes, and the phase-diverse speckle restoration algorithm was used to reconstruct a set of 30 frames from disjoint sets of 10 images spanning the collection interval. The figure shows every other reconstructed image. The 30 object restorations were aligned and made into a movie. For ease of viewing, the movie has been Fourier-interpolated in the time dimension by a factor of 5 for a total of 150 frames. The resulting restored time series of a plage (a magnetic region without sunspots) shows the highly dynamic photosphere at scales below 0.3", close to the diffraction limit of the telescope. There is a substantial amount of evolution within less than a minute, in particular small features, probably associated with magnetic fields, evolve rapidly. An interesting example is seen in the lower left section of the field of view of the two images in the lower left part of the figure. A granule is expanding and hits an elongated, small feature, which then increases in brightness by about 30% within less than a minute. The reason for this dramatic increase of brightness will be the subject of further data analysis. The images in the narrow-band channel will help to determine the location of magnetic fields. During the course of the observations, data sets under better seeing conditions spanned several hours. The very high spatial resolution combined with the good time resolution opens a new window to the study of the dynamics of the solar atmosphere from ground-based observatories. The movie as well as general information on phase-diverse speckle imaging has been made available on the World Wide Web at http://www.erim.org/algs/PD/pd_home.html. Christoph Keller
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