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Measuring the faint polarization signal of the cosmic microwave background(CMB) not only requires high optical throughput and instrument sensitivity butalso control over systematic effects. Polarimetric cameras or receivers used inthis setting often employ dielectric vacuum windows, filters, or lenses toappropriately prepare light for detection by cooled sensor arrays. Theseelements in the optical chain are typically designed to minimize reflectivelosses and hence improve sensitivity while minimizing potential imagingartifacts such as glint and ghosting. The Primordial Inflation PolarizationExploreR (PIPER) is a balloon-borne instrument designed to measure thepolarization of the CMB radiation at the largest angular scales andcharacterize astrophysical dust foregrounds. PIPER's twin telescopes anddetector 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 detectorarrays. Light passes from the skyward portions of the telescope to the detectorarrays though this window, which utilizes an indium seal to prevent superfluidhelium leaks into the vacuum cryostat volume. The AR coating implementedreduces reflections from each interface to <1% compared to ~10% from anuncoated window surface. The AR coating procedure and room temperature opticalmeasurements of the window are presented. The indium vacuum sealing process isalso described in detail and test results characterizing its integrity tosuperfluid helium leaks are provided.

Anti-reflection Coated Vacuum Window for the Primordial Inflation Polarization ExploreR (PIPER) balloon-borne instrument