Hayato TAKAKURA

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 degrees x 9 degrees 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 similar to 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.

© 2019 IEEE. Polarization of the cosmic microwave background (CMB) has crucial information on the inflationary universe. To detect these signals, it is necessary to suppress far sidelobes of a telescope, which contaminate the CMB signals with strong foreground radiation, such as the Galactic plane. LiteBIRD is the only funded CMB observation satellite for the 2020s, and the low frequency telescope (LFT; 34-161 GHz) is one of its telescopes. We measured near-field antenna patterns of the LFT using its 1/4-scaled model and examined far sidelobes up to 60° from the peaks. To cover the 20° field of view of the LFT, we investigated the antenna patterns at the edges of the focal plane as well as at the center. The measurement frequencies were 140-220 GHz, which correspond to the lowest bands (35-55 GHz) of the full-scale LFT. The measurements were consistent with the simulated far-sidelobe patterns at least -50 dB level, and showed that far sidelobes for two orthogonal polarization directions are consistent with each other down to -40 dB level. We also measured the cross-polarization patterns, and their peak level was less than -20 dB.