PSFs, Photometry, and Astrometry for the ACS/WFC

We develop and present effective PSFs for the F606W filter in the WFC, and then for five other filters. After briefly reviewing the concept of the effective PSF, we show that the WFC PSF varies with position in the detector. We represent the PSF by a 9×10 array of fiducial PSFs, between which a tailor-made PSF is to be interpolated for each star. Fitting these PSFs to star images gives photometry and astrometry with accuracies of 0.01 mag and 0.01 pixel, respectively, for well-exposed star images. We study short-term variability of the PSF and also study it over the history of ACS. The variability with time depends mainly on focus, and can be represented as a spatially fixed “perturbation PSF” for each image. We make FORTRAN programs available for generating our PSFs, for measuring star images with them, and for a number of related utility tasks.