Optical characterization of the BICEP array 150 and 220/270GHz CMB polarimeters in the 2026 season

Poza, M. Izquierdo, Ade, P. A. R. ORCID: https://orcid.org/0000-0002-5127-0401, Ahmed, Z., Amiri, M., Barkats, D., Thakur, R. Basu, Bischoff, C. A., Beck, D., Bock, J. J., Buza, V., Cantrall, B., Cheshire, J. R., Connors, J., Cornelison, J., Crumrine, M., Cukierman, A., Denison, E., Duband, L., Echter, M., Eiben, M., Elwood, B. D., Fatigoni, S., Filippini, J. P., Fortes, A., Gao, M., Giannakopoulos, C., Goeckner-Wald, N., Goldfinger, D. C., Grayson, J. A., Greathouse, A., Grimes, P. K., Halpern, M., Henderson, S., Hoang, T. D., Hubmayr, J., Hui, H., Irwin, K. D., Kang, J. H., Karkare, K. S., Kefeli, S., Kovac, J. M., Kuo, C., Lasko, K., Lau, K., Lautzenhiser, M., Liu, T., Mackey, S. C., Maher, N., Megerian, K. G., Minutolo, L., Moncelsi, L., Nakato, Y., Nguyen, H. T., O'Brient, R., Paine, S. N., Patel, A., Petroff, M. A., Polish, A. R., Prouve, T., Pryke, C., Reintsema, C. D., Romand, T., Salatino, M., Schillaci, A., Schmitt, B., Singari, B., Soliman, A., Germaine, T. St., Steiger, A., Steinbach, B., Sudiwala, R. ORCID: https://orcid.org/0000-0003-3240-5304, Thompson, K. L., Tucker, C. ORCID: https://orcid.org/0000-0002-1851-3918, Turner, A. D., Vergès, C., Vieregg, A. G., Wandui, A., Weber, A. C., Willmert, J., Wu, W. L. K., Yang, H., Yu, C., Zeng, L., Zhang, C. and Zhang, S. 2026. Optical characterization of the BICEP array 150 and 220/270GHz CMB polarimeters in the 2026 season. Presented at: SPIE Astronomical Telescopes + Instrumentation, 2026, Copenhagen, Denmark, 5-10 July 2026. Published in: Zmuidzinas, Jonas and Gao, Jian-Rong eds. Proceedings Volume 14156: Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XIII. Proceedings of SPIE. , vol.14156 Society of Photo-optical Instrumentation Engineers, 10.1117/12.3104091

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Abstract

BICEP Array (BA) is the current-generation instrument in the BICEP series of small-aperture, on-axis refracting telescopes at the South Pole, designed to constrain the tensor-to-scalar ratio r through degree-scale measurements of B-mode polarization in the cosmic microwave background (CMB). As BA pushes to deeper sensitivity, control of instrumental systematics, and beam shape mismatch between the co-located orthogonally polarized detectors in particular, has become an increasingly important factor in translating raw sensitivity into a robust constraint on r. In these proceedings we report on the 2026 far field beam mapping (FFBM) campaign, which used a thermal chopped source to characterize the BA2 (150 GHz) and BA3 (220/270 GHz) beams. We fit twodimensional elliptical Gaussians to each detector’s beam, derive per-pair differential parameters (differential pointing, beamwidth, and ellipticity). The resulting high-signal-to-noise array-averaged beam maps are used to compute the beam window function Bl for the power spectrum analysis, while the individual per-detector beams feed dedicated beam convolution simulations used to validate the temperature-to-polarization (T→ P) deprojection procedure. After correcting for the chopper aperture, the recovered beamwidths follow the expected λ/D ordering. We also describe two pipeline improvements carried out during the 2026 campaign: an out-andback jackknife for noise quantification and an elnod-based gain calibration.

Item Type:	Conference or Workshop Item - published (Paper)
Date Type:	Publication
Status:	Published
Schools:	Schools > Physical, Chemical & Environmental Sciences Schools > Physics and Astronomy
Additional Information:	Article number: 141561X
Publisher:	Society of Photo-optical Instrumentation Engineers
ISSN:	0277-786X
Last Modified:	27 Aug 2026 13:30
URI:	https://orca.cardiff.ac.uk/id/eprint/189214

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