Straylight identification of a crossed-Dragone telescope by time-gated near-field antenna pattern measurements

Hayato Takakura; Ryo Nakano; Yutaro Sekimoto; Junji Inatani; Masahiro Sugimoto; Frederick Matsuda; Shugo Oguri

doi:10.1117/12.2627421

27 August 2022 Straylight identification of a crossed-Dragone telescope by time-gated near-field antenna pattern measurements

Hayato Takakura, Ryo Nakano, Yutaro Sekimoto, Junji Inatani, Masahiro Sugimoto, Frederick Matsuda, Shugo Oguri

Author Affiliations +

Proceedings Volume 12180, Space Telescopes and Instrumentation 2022: Optical, Infrared, and Millimeter Wave; 1218052 (2022) https://doi.org/10.1117/12.2627421
Event: SPIE Astronomical Telescopes + Instrumentation, 2022, Montréal, Québec, Canada

Conference Poster

Abstract

Suppression of straylight is one of the challenges in the optical design of a wide-field-of-view telescope. It contaminates the weak target signal with radiation from strong sources at angles far from the observing direction. We evaluated the optical design of a crossed-Dragone telescope, the LiteBIRD Low-Frequency Telescope (LFT), which has 18° ×9° field of view. We measured a 1/4-scaled antenna of the LFT at accordingly scaled frequencies of 160–200 GHz (corresponding to 40–50 GHz for the full-scale LFT), for the feed at the center and the edges of the focal plane. To separate straylight components, we computed the time profiles of the aperture fields with ∼ 0.1 ns resolution by inverse Fourier transformation of the measured frequency spectra and applied time gating to them. We identified far-sidelobe components in the time-gated antenna beam patterns whose arrival time and angular direction are consistent with straylight predicted by a ray-tracing simulation. The identified far-sidelobe components include straylight reduced but reflected inside the front hood and straylight with multiple reflections without intercepted by the front hood. Their intensities are less than the −56 dB level, which is the far-sidelobe knowledge requirement for the LFT.

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Hayato Takakura, Ryo Nakano, Yutaro Sekimoto, Junji Inatani, Masahiro Sugimoto, Frederick Matsuda, and Shugo Oguri "Straylight identification of a crossed-Dragone telescope by time-gated near-field antenna pattern measurements", Proc. SPIE 12180, Space Telescopes and Instrumentation 2022: Optical, Infrared, and Millimeter Wave, 1218052 (27 August 2022); https://doi.org/10.1117/12.2627421

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