  We present a compact two-lens HDPE f/1.6 refractor design that is capable of
supporting a 28-deg diffraction-limited field of view at 1-mm wavelengths and
contrast it to a similar two-lens refractor using silicon lenses. We compare
the optical properties of these two systems as predicted by both geometrical
and physical optics. The presented analysis suggests that by relaxing
telecentricity requirements, a plastic two-lens refractor system can perform
comparably to a similar silicon system across a wide field of view and
wavelengths up to 1 mm. We show that for both telescope designs, cold stop
spillover changes significantly across the field of view in a way that is
somewhat inconsistent with Gaussian beam formalism and simple f-number scaling.
We present results that highlight beam ellipticity dependence on both pixel
location and pixel aperture size --- an effect that is challenging to reproduce
in standard geometrical optics. We show that a silicon refractor design suffers
from larger cross-polarization compared to the HDPE design. Our results address
the limitations of solely relying on geometrical optics to assess relative
performance of two optical systems. We discuss implications for future
refractor designs.
