The Primordial Inflation Polarization ExploreR ( PIPER ) is a balloon-borne telescope designed to measure the polarization of the Cosmic Microwave Background on large angular scales .
PIPER will map 85 % of the sky at 200 , 270 , 350 , and 600 GHz over a series of 8 conventional balloon flights from the northern and southern hemispheres .
The first science flight will use two 32 \times 40 arrays of backshort-under-grid transition edge sensors , multiplexed in the time domain , and maintained at 100 mK by a Continuous Adiabatic Demagnetization Refrigerator .
Front-end cryogenic Variable-delay Polarization Modulators provide systematic control by rotating linear to circular polarization at 3 Hz .
Twin telescopes allow PIPER to measure Stokes I , Q , U , and V simultaneously .
The telescope is maintained at 1.5 K in an LHe bucket dewar .
Cold optics and the lack of a warm window permit sensitivity at the sky-background limit .
The ultimate science target is a limit on the tensor-to-scalar ratio of r \sim 0.007 , from the reionization bump to l \sim 300 .
PIPER ’ s first flight will be from the Northern hemisphere , and overlap with the CLASS survey at lower frequencies .
We describe the current status of the PIPER instrument .