Optical Characterization of the BICEP3 CMB Polarimeter at the South Pole
  BICEP3 is a small-aperture refracting cosmic microwave background (CMB)
telescope designed to make sensitive polarization maps in pursuit of a
potential B-mode signal from inflationary gravitational waves. It is the latest
in the BICEP/Keck Array series of CMB experiments at the South Pole, which has
provided the most stringent constraints on inflation to date. For the 2016
observing season, BICEP3 was outfitted with a full suite of 2400 optically
coupled detectors operating at 95 GHz. In these proceedings we report on the
far field beam performance using calibration data taken during the 2015-2016
summer deployment season in situ with a thermal chopped source. We generate
high-fidelity per-detector beam maps, show the array-averaged beam profile, and
characterize the differential beam response between co-located, orthogonally
polarized detectors which contributes to the leading instrumental systematic in
pair differencing experiments. We find that the levels of differential
pointing, beamwidth, and ellipticity are similar to or lower than those
measured for BICEP2 and Keck Array. The magnitude and distribution of BICEP3's
differential beam mismatch - and the level to which temperature-to-polarization
leakage may be marginalized over or subtracted in analysis - will inform the
design of next-generation CMB experiments with many thousands of detectors.
