Optical characterization of the Keck Array and BICEP3 CMB Polarimeters from 2016 to 2019

• Main Authors: Collaboration, The BICEP Keck, Germaine, T St, Ade, PAR, Ahmed, Z, Amiri, M, Barkats, D, Thakur, R Basu, Bischoff, CA, Bock, JJ, Boenish, H, Bullock, E, Buza, V, Cheshire, J, Connors, J, Cornelison, J, Crumrine, M, Cukierman, A, Dierickx, M, Duband, L, Fatigoni, S, Filippini, JP, Fliescher, S, Grayson, JA, Hall, G, Halpern, M, Harrison, S, Hildebrandt, SR, Hilton, GC, Hui, H, Irwin, KD, Kang, J, Karkare, KS, Karpel, E, Kefeli, S, Kernasovskiy, SA, Kovac, JM, Kuo, CL, Lau, K, Leitch, EM, Megerian, KG, Moncelsi, L, Namikawa, T, Netterfield, CB, Nguyen, HT, O'Brient, R, IV, RW Ogburn, Palladino, S, Pryke, C, Racine, B, Reintsema, CD, Richter, S, Schillaci, A, Schwarz, R, Sheehy, CD, Soliman, A, Steinbach, B, Sudiwala, RV, Thompson, KL, Tolan, JE, Tucker, C, Turner, AD, Umilta, C, Vieregg, AG, Wandui, A, Weber, AC, Wiebe, DV, Willmert, J, Wong, CL, Wu, WLK, Yang, E, Yoon, KW, Young, E, Yu, C, Zhang, C
• Format: Article in Journal/Newspaper
• Language: English
• Published: Springer Nature 2020
• Subjects: Cosmic microwave background; Inflation; Polarization; Transition-edge sensor; Keck Array; BICEP3; South Pole; South pole
• Online Access: https://www.repository.cam.ac.uk/handle/1810/302913; https://doi.org/10.17863/CAM.49988

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.