Optical characterization of the Keck Array and BICEP3 CMB Polarimeters from 2016 to 2019
Creators: name: Keck Collaboration
name: Germaine, T. St
name: Ade, P. A. R.
name: Ahmed, Z.
name: Amiri, M.
name: Barkats, D.
name: Thakur, R. Basu
name: Bischoff, C. A.
name: Bock, J. J.
name: Boenish, H.
name: Bullock, E.
name: Buza, V.
name: Cheshire, J.
name: Connors, J.
name: Cornelison, J.
name: Crumrine, M.
name: Cukierman, A.
name: Dierickx, M.
name: Duband, L.
name: Fatigoni, S.
name: Filippini, J. P.
name: Fliescher, S.
name: Grayson, J. A.
name: Hall, G.
name: Halpern, M.
name: Harrison, S.
name: Hildebrandt, S. R.
name: Hilton, G. C.
name: Hui, H.
name: Irwin, K. D.
name: Kang, J.
name: Karkare, K. S.
name: Karpel, E.
name: Kefeli, S.
name: Kernasovskiy, S. A.
name: Kovac, J. M.
name: Kuo, C. L.
name: Lau, K.
name: Leitch, E. M.
name: Megerian, K. G.
name: Moncelsi, L.
name: Namikawa, T.
name: Netterfield, C. B.
name: Nguyen, H. T.
name: O'Brient, R.
name: Ogburn, R. W.
name: Palladino, S.
name: Pryke, C.
name: Racine, B.
name: Reintsema, C. D.
name: Richter, S.
name: Schillaci, A.
name: Schwarz, R.
name: Sheehy, C. D.
name: Soliman, A.
name: Steinbach, B.
name: Sudiwala, R. V.
name: Thompson, K. L.
name: Tolan, J. E.
name: Tucker, C.
name: Turner, A. D.
name: Umilta, C.
name: Vieregg, A. G.
name: Wandui, A.
name: Weber, A. C.
name: Wiebe, D. V.
name: Willmert, J.
name: Wong, C. L.
name: Wu, W. L. K.
name: Yang, E.
name: Yoon, K. W.
name: Young, E.
name: Yu, C.
name: Zhang, C.
Publisher: name: arXiv
Year: 2020
Subjects: subject: Instrumentation and Methods for Astrophysics (astro-ph.IM)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: The BICEP/Keck experiment (BK) is a series of small-aperture refracting telescopes observing degree-scale Cosmic Microwave Background (CMB) polarization from the South Pole in search of a primordial $B$-mode signature. This $B$-mode signal arises from primordial gravitational waves interacting with the CMB, and has amplitude parametrized by the tensor-to-scalar ratio $r$. Since 2016, BICEP3 and the Keck Array have been observing with 4800 total antenna-coupled transition-edge sensor detectors, with frequency bands spanning 95, 150, 220, and 270 GHz. Here we present the optical performance of these receivers from 2016 to 2019, including far-field beams measured in situ with an improved chopped thermal source and instrument spectral response measured with a field-deployable Fourier Transform Spectrometer. As a pair differencing experiment, an important systematic that must be controlled is the differential beam response between the co-located, orthogonally polarized detectors. We generate per-detector far-field beam maps and the corresponding differential beam mismatch that is used to estimate the temperature-to-polarization leakage in our CMB maps and to give feedback on detector and optics fabrication. The differential beam parameters presented here were estimated using improved low-level beam map analysis techniques, including efficient removal of non-Gaussian noise as well as improved spatial masking. These techniques help minimize systematic uncertainty in the beam analysis, with the goal of constraining the bias on $r$ induced by temperature-to-polarization leakage to be subdominant to the statistical uncertainty. This is essential as we progress to higher detector counts in the next generation of CMB experiments.
description: 8 pages, 3 figures. Accepted by the Journal of Low Temperature Physics (Proceedings of the 18th International Workshop on Low Temperature Detectors)