  Measuring the faint polarization signal of the cosmic microwave background
(CMB) not only requires high optical throughput and instrument sensitivity but
also control over systematic effects. Polarimetric cameras or receivers used in
this setting often employ dielectric vacuum windows, filters, or lenses to
appropriately prepare light for detection by cooled sensor arrays. These
elements in the optical chain are typically designed to minimize reflective
losses and hence improve sensitivity while minimizing potential imaging
artifacts such as glint and ghosting. The Primordial Inflation Polarization
ExploreR (PIPER) is a balloon-borne instrument designed to measure the
polarization of the CMB radiation at the largest angular scales and
characterize astrophysical dust foregrounds. PIPER's twin telescopes and
detector systems are submerged in an open-aperture liquid helium bucket dewar.
A fused-silica window anti-reflection (AR) coated with polytetrafluoroethylene
(PTFE) is installed on the vacuum cryostat that houses the cryogenic detector
arrays. Light passes from the skyward portions of the telescope to the detector
arrays though this window, which utilizes an indium seal to prevent superfluid
helium leaks into the vacuum cryostat volume. The AR coating implemented
reduces reflections from each interface to <1% compared to ~10% from an
uncoated window surface. The AR coating procedure and room temperature optical
measurements of the window are presented. The indium vacuum sealing process is
also described in detail and test results characterizing its integrity to
superfluid helium leaks are provided.
