The Primordial Inflation Polarization Explorer (PIPER): 2019 Flight and Telescope Performance

The Primordial Inflation Polarization Explorer (PIPER) is a balloon-borne instrument that will probe the epoch of reionization and search for the signature of inflation through large angular scale measurements of the linear polarization of the cosmic microwave background in four frequency bands from 200 to 600 GHz. PIPER consists of co-pointed twin cryogenic telescopes operating in an open liquid helium bucket dewar that upon achieving float altitude cool below a temperature of 3 K. Variable-delay polarization modulators (VPMs) provide sensitivity to circular polarization for systematic error control, along with one linear polarization component per telescope. The twin telescopes provide simultaneous sensitivity to Stokes I, Q, U, and V. On 14 October 2019, PIPER flew from Fort Sumner, NM, configured to observe in a band centered at 200 GHz. The flight lasted 13 hours, reaching an altitude of 30.2 km. Failure of an observation hatch to open upon reaching float altitude prevented astrophysical observations; however, the VPMs and telescope cryogenics were operated successfully. Here we report on the 2019 PIPER flight and the performance of these systems at float.