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

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.

The BICEP/Keck projects have been made possible through a series of Grants from the National Science Foundation including 0742818, 0742592, 1044978, 1110087, 1145172, 1145143, 1145248, 1639040, 1638957, 1638978, 1638970, and 1726917, by the Gordon and Betty Moore Foundation, by the Keck Foundation, and by the Grant 55802 from John Templeton Foundation. The development of antenna-coupled detector technology was supported by the JPL Research and Technology Development Fund and NASA Grants 06-ARPA206-0040, 10-SAT10-0017, 12-SAT12-0031, 14-SAT14-0009 and 16-SAT16-0002. The development and testing of focal planes were supported by the Gordon and Betty Moore Foundation at Caltech. Readout electronics were supported by a Canada Foundation for Innovation grant to UBC. The computations in this paper were run on the Odyssey cluster supported by the FAS Science Division Research Computing Group at Harvard University. The analysis effort at Stanford and SLAC is partially supported by the U.S. DoE Office of Science. We thank the staff of the U.S. Antarctic Program and in particular the South Pole Station without whose help this research would not have been possible. We thank all those who have contributed past efforts to the BICEP/Keck series of experiments, including the Bicep1 team. Tireless administrative support was provided by Kathy Deniston, Sheri Stoll, Irene Coyle, Donna Hernandez, Dana Volponi, and Julie Shih.

Authors and Affiliations

1. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, 02138, USA

   T. St Germaine, D. Barkats, H. Boenish, V. Buza, J. Connors, J. Cornelison, M. Dierickx, G. Hall, S. Harrison, K. S. Karkare, J. M. Kovac, B. Racine, S. Richter & C. L. Wong

2. School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA, UK

   P. A. R. Ade, R. V. Sudiwala & C. Tucker

3. Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, CA, 94025, USA

   Z. Ahmed, A. Cukierman, K. D. Irwin & E. Yang

4. Department of Physics, Stanford University, Stanford, CA, 94305, USA

   Z. Ahmed, A. Cukierman, J. A. Grayson, K. D. Irwin, J. Kang, E. Karpel, S. A. Kernasovskiy, C. L. Kuo, R. W. Ogburn IV, K. L. Thompson, J. E. Tolan, E. Yang, K. W. Yoon, E. Young & C. Yu

5. Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada

   M. Amiri, S. Fatigoni, M. Halpern & D. V. Wiebe

6. Department of Physics, California Institute of Technology, Pasadena, CA, 91125, USA

   R. Basu Thakur, J. J. Bock, H. Hui, S. Kefeli, L. Moncelsi, H. T. Nguyen, R. O’Brient, A. Schillaci, A. Soliman, B. Steinbach, A. Wandui & C. Zhang

7. Department of Physics, University of Cincinnati, Cincinnati, OH, 45221, USA

   C. A. Bischoff, S. Palladino & C. Umiltà

8. Jet Propulsion Laboratory, Pasadena, CA, 91109, USA

   J. J. Bock, S. R. Hildebrandt, K. G. Megerian, H. T. Nguyen, R. O’Brient, A. D. Turner & A. C. Weber

9. Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, 55455, USA

   E. Bullock, J. Cheshire, M. Crumrine, S. Fliescher, G. Hall, K. Lau, C. Pryke, R. Schwarz & J. Willmert

10. Service des Basses Températures, Commissariat à lEnergie Atomique, 38054, Grenoble, France

    L. Duband

11. Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA

    J. P. Filippini

12. Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA

    J. P. Filippini

13. National Institute of Standards and Technology, Boulder, CO, 80305, USA

    G. C. Hilton & C. D. Reintsema

14. Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL, 60637, USA

    K. S. Karkare, E. M. Leitch, A. G. Vieregg & W. L. K. Wu

15. Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge, CB3 0WA, UK

    T. Namikawa

16. Department of Physics, University of Toronto, Toronto, ON, M5S 1A7, Canada

    C. B. Netterfield

17. Canadian Institute for Advanced Research, Toronto, ON, M5G 1Z8, Canada

    C. B. Netterfield

18. Physics Department, Brookhaven National Laboratory, Upton, NY, 11973, USA

    C. D. Sheehy

Corresponding author

Correspondence to T. St Germaine.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Reprints and permissions