  Anisotropy in the point spread function (PSF) contributes a systematic error
to weak lensing measurements. In this study we use a ray tracer that
incorporates all the optical elements of the SNAP telescope to estimate this
effect. Misalignments in the optics generates PSF anisotropy, which we
characterize by its ellipticity. The effect of three time varying effects:
thermal drift, guider jitter, and structural vibration on the PSF are estimated
for expected parameters of the SNAP telescope. Multiple realizations of a
thousand square degree mock survey are then generated to include the systematic
error pattern induced by these effects. We quantify their contribution to the
power spectrum of the lensing shear. We find that the dominant effect comes
from the thermal drift, which peaks at angular wavenumbers l ~ 10^3, but its
amplitude is over one order of magnitude smaller than the size of the expected
statistical error. While there are significant uncertainties in our modeling,
our study indicates that time-varying PSFs will contribute at a smaller level
than statistical errors in SNAP's weak lensing measurements.
