  Forthcoming space-based observations will require high-quality point-spread
function (PSF) models for weak gravitational lensing measurements. One approach
to generating these models is using a wavefront model based on the known
telescope optics. We present an empirical framework for validating such models
to confirm that they match the actual PSF to within requirements by comparing
the models to the observed 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), testing 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 configuration, differ significantly from the observed
PSFs, most notably in their sizes. We find that the quality of Tiny Tim PSFs
can be improved through fitting the full set of Zernike polynomial coefficients
which characterise the optics, to the point where the practical significance of
the difference between model and observed PSFs is negligible for most use
cases, resulting in additive and multiplicative biases both of order
approximately 4e-4. We also show that most of this improvement can be retained
through using an updated set of Zernike coefficients, which we provide.
