LiteBIRD is a JAXA-led international project aimed at measuring the cosmic microwave background (CMB) polarization with high sensitivity to detect polarization B modes. This detection would provide evidence of inflation. LiteBIRD will observe the full sky for three years at the L2 Lagrange point of the Earth-Sun system across 34-448 GHz, and is expected to launch in the Japanese fiscal year of 2032. The Low-Frequency Telescope (LFT) will observe in the 34-161 GHz range implementing a modified crossed Dragone (MCD) reflective optical design optimized for high optical performance across a wide 18∘×9∘ field-of-view (FOV). In this paper, we report the LFT optical design details including its optimization and optical performance assessed using optical simulations. The MCD design consists of a paraboloidal primary and a hyperboloidal secondary reflector with polynomial correction terms up to 7th order, achieving Strehl ratios ≥0.97 at 161 GHz across the FOV. The Mueller QU (UQ) cross-polarization response is ≤−26.9 dB at 34 GHz. The simulated beam sizes are <78′ at 34 GHz. The simulated sidelobe response for the direct and diffuse triple reflection sidelobes are estimated to be <−57 dB, and for the focused triple reflection sidelobe <−37 dB at 34 GHz. The LFT optical design satisfies all the optical requirements and specifications for the project, and is compatible with the LiteBIRD science goals.
@article{arxiv.2505.15102,
title = {Modified crossed Dragone optical design of the LiteBIRD low-frequency telescope},
author = {Frederick Matsuda and Shugo Oguri and Yutaro Sekimoto and Aritoki Suzuki and Hayato Takakura and Shingo Kashima},
journal= {arXiv preprint arXiv:2505.15102},
year = {2025}
}