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

The point-spread function (PSF) of an adaptive optics (AO) system is oftenpoorly known. This ignorance can lead to significant systematic errors inphotometry. Since the degree of AO correction is sensitive to the observingconditions: seeing, wind speed, brightness of the wavefront reference, etc., itwould be desirable to estimate the PSF from the data themselves. We havedeveloped a method to estimate the PSF delivered by an AO system in the casewhere the scene consists of a crowded star field. We model the modulationtransfer function (MTF) of several key components of the imaging system. Thepower spectrum of the image, even a dense star field, can then be used toconstrain our model, which in turn can be used to reconstruct the PSF. We showthat the method yields reasonable fit parameters and a useful approximation tothe PSF when applied to data from laser guide star (LGS) AO system at the KeckObservatory. Comparison of Keck LGS/AO and HST/NICMOS data shows thatphotometric accuracy of a few percent can be achieved for data with Strehlratios as low as 4%.Comment: 19 pages & 14 figures. Accepted for publication in Astrophysical Journal. Based on observations obtained at the W.M. Keck Observator