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Yamada K, Bixler B, Sakurai Y, Ashton PC, Sugiyama J, Arnold K, Begin J, Corbett L, Day-Weiss S, Galitzki N, Hill CA, Johnson BR, Jost B, Kusaka A, Koopman BJ, Lashner J, Lee AT, Mangu A, Nishino H, Page LA, Randall MJ, Sasaki D, Song X, Spisak J, Tsan T, Wang Y, Williams PA. The Simons Observatory: Cryogenic half wave plate rotation mechanism for the small aperture telescopes. Rev Sci Instrum 2024; 95:024504. [PMID: 38385955 DOI: 10.1063/5.0178066] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/26/2023] [Accepted: 02/01/2024] [Indexed: 02/23/2024]
Abstract
We present the requirements, design, and evaluation of the cryogenic continuously rotating half-wave plate (CHWP) for the Simons Observatory (SO). SO is a cosmic microwave background polarization experiment at Parque Astronómico de Atacama in northern Chile that covers a wide range of angular scales using both small (⌀0.42 m) and large (⌀6 m) aperture telescopes. In particular, the small aperture telescopes (SATs) focus on large angular scales for primordial B-mode polarization. To this end, the SATs employ a CHWP to modulate the polarization of the incident light at 8 Hz, suppressing atmospheric 1/f noise and mitigating systematic uncertainties that would otherwise arise due to the differential response of detectors sensitive to orthogonal polarizations. The CHWP consists of a 505 mm diameter achromatic sapphire HWP and a cryogenic rotation mechanism, both of which are cooled down to ∼50 K to reduce detector thermal loading. Under normal operation, the HWP is suspended by a superconducting magnetic bearing and rotates with a constant 2 Hz frequency, controlled by an electromagnetic synchronous motor. We find that the number of superconductors and the number of magnets that make up the superconducting magnetic bearing are important design parameters, especially for the rotation mechanism's vibration performance. The rotation angle is detected through an angular encoder with a noise level of 0.07 μrad s. During a cooldown process, the rotor is held in place by a grip-and-release mechanism that serves as both an alignment device and a thermal path. In this paper, we provide an overview of the SO SAT CHWP: its requirements, hardware design, and laboratory performance.
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Affiliation(s)
- K Yamada
- Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
| | - B Bixler
- Department of Physics, University of California, San Diego, La Jolla, California 92093, USA
| | - Y Sakurai
- Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
- Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, Chiba 277-8583, Japan
| | - P C Ashton
- Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, Chiba 277-8583, Japan
- Physics Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
- Department of Physics, University of California, Berkeley, California 94720, USA
| | - J Sugiyama
- Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
| | - K Arnold
- Department of Physics, University of California, San Diego, La Jolla, California 92093, USA
| | - J Begin
- Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, New Jersey 08544, USA
| | - L Corbett
- Physics Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
- Department of Physics, University of California, Berkeley, California 94720, USA
| | - S Day-Weiss
- Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, New Jersey 08544, USA
| | - N Galitzki
- Department of Physics, University of Texas at Austin, Austin, Texas 78722, USA
- Weinberg Institute for Theoretical Physics, Texas Center for Cosmology and Astroparticle Physics, Austin, Texas 78712, USA
| | - C A Hill
- Physics Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
- Department of Physics, University of California, Berkeley, California 94720, USA
| | - B R Johnson
- Department of Astronomy, University of Virginia, Charlottesville, Virginia 22904, USA
| | - B Jost
- Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, Chiba 277-8583, Japan
| | - A Kusaka
- Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
- Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, Chiba 277-8583, Japan
- Physics Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - B J Koopman
- Wright Laboratory, Department of Physics, Yale University, New Haven, Connecticut 06520, USA
| | - J Lashner
- Wright Laboratory, Department of Physics, Yale University, New Haven, Connecticut 06520, USA
| | - A T Lee
- Physics Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
- Department of Physics, University of California, Berkeley, California 94720, USA
| | - A Mangu
- Department of Physics, University of California, Berkeley, California 94720, USA
| | - H Nishino
- Research Center for the Early Universe, School of Science, The University of Tokyo, Tokyo 113-0033, Japan
| | - L A Page
- Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, New Jersey 08544, USA
| | - M J Randall
- Department of Physics, University of California, San Diego, La Jolla, California 92093, USA
| | - D Sasaki
- Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
| | - X Song
- Department of Physics, University of California, Berkeley, California 94720, USA
| | - J Spisak
- Department of Physics, University of California, San Diego, La Jolla, California 92093, USA
| | - T Tsan
- Department of Physics, University of California, San Diego, La Jolla, California 92093, USA
| | - Y Wang
- Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, New Jersey 08544, USA
| | - P A Williams
- Physics Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
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