Optical characterization of the Keck Array and BICEP3 CMB Polarimeters from 2016 to 2019. (arXiv:2002.05197v1 [astro-ph.IM])

Optical characterization of the Keck Array and BICEP3 CMB Polarimeters from 2016 to 2019. (arXiv:2002.05197v1 [astro-ph.IM])
The <a href="http://arxiv.org/find/astro-ph/1/au:+Collaboration_BICEP/Keck/0/1/0/all/0/1">BICEP/Keck Collaboration</a>: <a href="http://arxiv.org/find/astro-ph/1/au:+Germaine_T/0/1/0/all/0/1">T. St Germaine</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ade_P/0/1/0/all/0/1">P. A. R. Ade</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ahmed_Z/0/1/0/all/0/1">Z. Ahmed</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Amiri_M/0/1/0/all/0/1">M. Amiri</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Barkats_D/0/1/0/all/0/1">D. Barkats</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Thakur_R/0/1/0/all/0/1">R. Basu Thakur</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bischoff_C/0/1/0/all/0/1">C. A. Bischoff</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bock_J/0/1/0/all/0/1">J. J. Bock</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Boenish_H/0/1/0/all/0/1">H. Boenish</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bullock_E/0/1/0/all/0/1">E. Bullock</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Buza_V/0/1/0/all/0/1">V. Buza</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cheshire_J/0/1/0/all/0/1">J. Cheshire</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Connors_J/0/1/0/all/0/1">J. Connors</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cornelison_J/0/1/0/all/0/1">J. Cornelison</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Crumrine_M/0/1/0/all/0/1">M. Crumrine</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cukierman_A/0/1/0/all/0/1">A. Cukierman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dierickx_M/0/1/0/all/0/1">M. Dierickx</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Duband_L/0/1/0/all/0/1">L. Duband</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fatigoni_S/0/1/0/all/0/1">S. Fatigoni</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Filippini_J/0/1/0/all/0/1">J. P. Filippini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fliescher_S/0/1/0/all/0/1">S. Fliescher</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Grayson_J/0/1/0/all/0/1">J. A. Grayson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hall_G/0/1/0/all/0/1">G. Hall</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Halpern_M/0/1/0/all/0/1">M. Halpern</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Harrison_S/0/1/0/all/0/1">S. Harrison</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hildebrandt_S/0/1/0/all/0/1">S. R. Hildebrandt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hilton_G/0/1/0/all/0/1">G. C. Hilton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hui_H/0/1/0/all/0/1">H. Hui</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Irwin_K/0/1/0/all/0/1">K. D. Irwin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kang_J/0/1/0/all/0/1">J. Kang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Karkare_K/0/1/0/all/0/1">K. S. Karkare</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Karpel_E/0/1/0/all/0/1">E. Karpel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kefeli_S/0/1/0/all/0/1">S. Kefeli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kernasovskiy_S/0/1/0/all/0/1">S. A. Kernasovskiy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kovac_J/0/1/0/all/0/1">J. M. Kovac</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kuo_C/0/1/0/all/0/1">C. L. Kuo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lau_K/0/1/0/all/0/1">K. Lau</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leitch_E/0/1/0/all/0/1">E. M. Leitch</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Megerian_K/0/1/0/all/0/1">K. G. Megerian</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Moncelsi_L/0/1/0/all/0/1">L. Moncelsi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Namikawa_T/0/1/0/all/0/1">T. Namikawa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Netterfield_C/0/1/0/all/0/1">C. B. Netterfield</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nguyen_H/0/1/0/all/0/1">H. T. Nguyen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+OBrient_R/0/1/0/all/0/1">R. O'Brient</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ogburn_R/0/1/0/all/0/1">R.W. Ogburn IV</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Palladino_S/0/1/0/all/0/1">S. Palladino</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pryke_C/0/1/0/all/0/1">C. Pryke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Racine_B/0/1/0/all/0/1">B. Racine</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reintsema_C/0/1/0/all/0/1">C. D. Reintsema</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Richter_S/0/1/0/all/0/1">S. Richter</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schillaci_A/0/1/0/all/0/1">A. Schillaci</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Schwarz_R/0/1/0/all/0/1">R. Schwarz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sheehy_C/0/1/0/all/0/1">C. D. Sheehy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Soliman_A/0/1/0/all/0/1">A. Soliman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Steinbach_B/0/1/0/all/0/1">B. Steinbach</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sudiwala_R/0/1/0/all/0/1">R. V. Sudiwala</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Thompson_K/0/1/0/all/0/1">K. L. Thompson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tolan_J/0/1/0/all/0/1">J. E. Tolan</a>, et al. (15 additional authors not shown)

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.

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