Paper: astro-ph/0604551
Date: Wed, 26 Apr 2006 20:00:17 GMT (363kb)

Title: Constraining the Adaptive Optics Point-Spread Function in Crowded
Fields: Measuring Photometric Aperture Corrections

Authors: Christopher D. Sheehy (1), Nate McCrady (1,2) and James R. Graham (1)
((1) University of California, Berkeley, (2) UCLA)

Comments: 19 pages & 14 figures. Accepted for publication in Astrophysical
Journal. Based on observations obtained at the W.M. Keck Observatory
\\
The point-spread function (PSF) of an adaptive optics (AO) system is often
poorly known. This ignorance can lead to significant systematic errors in
photometry. Since the degree of AO correction is sensitive to the observing
conditions: seeing, wind speed, brightness of the wavefront reference, etc., it
would be desirable to estimate the PSF from the data themselves. We have
developed a method to estimate the PSF delivered by an AO system in the case
where the scene consists of a crowded star field. We model the modulation
transfer function (MTF) of several key components of the imaging system. The
power spectrum of the image, even a dense star field, can then be used to
constrain our model, which in turn can be used to reconstruct the PSF. We show
that the method yields reasonable fit parameters and a useful approximation to
the PSF when applied to data from laser guide star (LGS) AO system at the Keck
Observatory. Comparison of Keck LGS/AO and HST/NICMOS data shows that
photometric accuracy of a few percent can be achieved for data with Strehl
ratios as low as 4%.

\\ ( http://arXiv.org/abs/astro-ph/0604551 , 363kb)