Evaluation of reconstructed angular error of a continuous rotating HWP for LiteBIRD

Shinya Sugiyama; Tomotake Matsumura; Yuki Sakurai; Nobuhiko Katayama; Satoru Takakura; Makoto Tashiro; Yukikatsu Terada; Kosuke Sato; Satoru Katsuda; Yurika Hoshino; Ryota Takaku; Kunimoto Komatsu

doi:10.1117/12.2576290

15 December 2020 Evaluation of reconstructed angular error of a continuous rotating HWP for LiteBIRD

Shinya Sugiyama, Tomotake Matsumura, Yuki Sakurai, Nobuhiko Katayama, Satoru Takakura, Makoto Tashiro, Yukikatsu Terada, Kosuke Sato, Satoru Katsuda, Yurika Hoshino, Ryota Takaku, Kunimoto Komatsu

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Proceedings Volume 11443, Space Telescopes and Instrumentation 2020: Optical, Infrared, and Millimeter Wave; 114436X (2020) https://doi.org/10.1117/12.2576290
Event: SPIE Astronomical Telescopes + Instrumentation, 2020, Online Only

Conference Poster

Abstract

We report the development of an optical encoder and its readout system for a cryogenically-cooled continuously rotating half-wave plate (HWP) polarization modulator unit (PMU) in the LiteBIRD low-frequency telescope. LiteBIRD is a cosmic microwave background polarization satellite mission to probe B-mode polarization, which originates from primordial gravitational waves, observing from the second Lagrange point (L2). LiteBIRD employs a continuously-rotating HWP to mitigate systematic effects. The knowledge of the position angle of the HWP is in a one-to-one relationship to the incident polarization angle. The required reconstruction accuracy is about 1 arcmin and the targeted rotational frequency stability is 1 mHz. A unique development constraint comes from a telemetry bandwidth limitation between the Earth and L2, and thus we implement a digital process to reduce the data volume assuming a future implementation of on-board processing of the encoder data before the downlink. The demonstrations were done experimentally using a breadboard model of the PMU: a readout system using FPGA (Spartan-6) and a rotational mechanism using a superconducting magnetic bearing and AC motor. We acquired the encoder data from the rotational mechanism operating under two conditions: liquid nitrogen at room pressure and below 10 K in a cryostat. We demonstrated the reconstruction of the position angle accuracy < 0.5 arcmin and the corresponding data volume of 0.12 GB/day, which is at least an order of magnitude smaller than the total data volume per day. We further discuss the sources of the position angle uncertainty and its implications to the observations.

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Shinya Sugiyama, Tomotake Matsumura, Yuki Sakurai, Nobuhiko Katayama, Satoru Takakura, Makoto Tashiro, Yukikatsu Terada, Kosuke Sato, Satoru Katsuda, Yurika Hoshino, Ryota Takaku, and Kunimoto Komatsu "Evaluation of reconstructed angular error of a continuous rotating HWP for LiteBIRD", Proc. SPIE 11443, Space Telescopes and Instrumentation 2020: Optical, Infrared, and Millimeter Wave, 114436X (15 December 2020); https://doi.org/10.1117/12.2576290

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