Optical Characterization of the BICEP3 CMB Polarimeter at the South Pole

TL;DR

BICEP3 is a sensitive CMB polarization telescope that characterizes its optical performance to improve measurements of inflationary gravitational waves, with detailed beam analysis informing future experiment designs.

Contribution

This paper provides the first detailed optical characterization of BICEP3's beam performance, including differential beam response, at 95 GHz during the 2015-2016 season.

Findings

Differential pointing, beamwidth, and ellipticity are comparable or lower than previous BICEP/Keck experiments.

High-fidelity per-detector beam maps were generated using in situ calibration data.

Results inform the design of next-generation CMB experiments with larger detector arrays.

Abstract

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.