  In weak lensing investigations, galaxy shapes are deconvolved for the effects
of the point spread function (PSF) using stellar images. In this paper we use
physical models of the telescope optics to understand the spatial variation of
the PSF in the image plane. We introduce a set of parameters to model the key
aberrations, which include defocus, focal plane tilt, primary and off-axis
astigmatism. We also include the effects of guiding and seeing. We test our
model with data from the Blanco 4 meter telescope in Cerro Tololo, Chile. We
find that the physical model describes a substantial part of the PSF size and
anisotropy over the field of view (over 90 percent of it, based on a
chi-squared metric). We identify the primary contributors to the PSF patterns
and study their covariances and principal components. We also identify
correlations with the effect of gravity on the telescope. Finally, we discuss
the improvements in PSF estimation that may be achieved by combining the
physical model in this study with the purely empirical approach of Jarvis and
Jain (2004).
