Validation of PSF Models for HST and Other Space-Based Observations

Forthcoming space-based observations will require high-quality point-spread function (PSF) models for weak gravitational lensing measurements. One approach to gener- ating these models is using a wavefront model based on the known telescope optics. We present an empirical framework for validating such models to conrm that they match the actual PSF to within requirements by comparing the models to the ob- served light distributions of isolated stars. We apply this framework to Tiny Tim, the standard tool for generating model PSFs for the Hubble Space Telescope (HST), test- ing its models against images taken by HST's Advanced Camera for Surveys in the Wide Field Channel. We show that Tiny Tim's models, in the default conguration, dier signicantly from the observed PSFs, most notably in their sizes. We nd that the quality of Tiny Tim PSFs can be improved through tting the full set of Zernike polynomial coecients which characterise the optics, to the point where the practical signicance of the dierence between model and observed PSFs is negligible for most use cases, resulting in additive and multiplicative biases both of order 410􀀀4. We also show that most of this improvement can be retained through using an updated set of Zernike coefficients, which we provide.