  Star test polarimetry is an imaging polarimetry technique in which an element
with spatially-varying birefringence is placed in the pupil plane to encode
polarization information into the point-spread function (PSF) of an imaging
system. In this work, a variational calculation is performed to find the
optimal birefringence distribution that effectively encodes polarization
information while producing the smallest possible PSF, thus maximizing the
resolution for imaging polarimetry. This optimal solution is found to be nearly
equivalent to the birefringence distribution that results from a glass window
being subjected to three uniformly spaced stress points at its edges, which has
been used in previous star test polarimetry setups.
