Physics – Optics
Scientific paper
May 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003phdt.........1w&link_type=abstract
PhD Thesis, Naturwissenschaftlich-Mathematische Gesamtfakultät der Universität Heidelberg, Germany. II + 105 pp. (2003)
Physics
Optics
5
Adaptive Optics Systems, Point Spread Functions, Photometry
Scientific paper
The ALFA adaptive optics (AO) system, operated by the Max-Planck-Institut für Astronomie at the Calar Alto Observatory, has been available as a standard facility instrument for some years now. As with other AO systems, however, the scientific use of observations with this instrument is complicated by the time- and space-variant nature of the system point spread function (PSF). Generally, the PSF information cannot be taken from the images due to anisoplanacy and/or overexposure of the guide star. It is therefore highly desirable to obtain an independent estimate of the PSF at each position in the field of view. This estimate can be derived from the wavefront sensor (WFS) signals of the ALFA system. It was necessary to adapt an existing algorithm developed for curvature sensors to the Shack-Hartmann sensor (SHS) used on ALFA. The practicability and performance of this algorithm was then tested using simulations of atmospheric turbulence. WFS signals are affected by noise, depending on the brightness of the guide star as well as the selected loop frequency; this noise has to be reliably determined in order to obtain satisfactory results from the PSF estimation process. With ALFA, noise estimation is only possible indirectly by time-series analysis of the WFS signals. A new method is proposed that delivers superior results for faint guide stars and low loop frequencies. PSF estimation away from the guide star requires knowledge of the vertical turbulence profile of the atmosphere. Hence, SCIDAR measurements were carried out alongside AO observations. These measurements also served to cross-check estimates of atmospheric parameters gained from WFS signals as well as images. Finally, on- and off-axis PSFs extracted from images were compared to their estimated counterparts, with good agreement found in all cases. Additionally, it could be shown that the photometric accuracy is strongly improved by using a locally estimated PSF as compared to the guide star PSF.
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