Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26as..107..243l&link_type=abstract
Astronomy and Astrophysics Suppl. 107, 243-264 (1994)
Astronomy and Astrophysics
Astrophysics
69
Atmospheric Effects, Methods: Numerical, Methods: Observational, Techniques: Interferometric, Telescopes, Sun: General Year: 1994
Scientific paper
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.
Löfdahl Mats G.
Scharmer Goeran B.
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