Shugo Oguri

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 seventh order, achieving Strehl ratios ≥0.97 at 161 GHz across the FOV. The Mueller QU (UQ) cross-polarization response is ≤−26.9dB at 34 GHz. The simulated beam sizes are <78^′ at 34 GHz. The simulated sidelobe response for the direct and diffuse triple reflection sidelobes is estimated to be <−57dB and for the focused triple reflection sidelobe <−37dB 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.

<jats:title>Abstract</jats:title> <jats:p>Large angular scale surveys in the absence of atmosphere are essential for measuring the primordial B-mode power spectrum of the Cosmic Microwave Background (CMB). Since this proposed measurement is about three to four orders of magnitude fainter than the temperature anisotropies of the CMB, in-flight calibration of the instruments and active suppression of systematic effects are crucial. We investigate the effect of changing the parameters of the scanning strategy on the in-flight calibration effectiveness, the suppression of the systematic effects themselves, and the ability to distinguish systematic effects by null-tests. Next-generation missions such as <jats:italic>LiteBIRD</jats:italic>, modulated by a Half-Wave Plate (HWP), will be able to observe polarisation using a single detector, eliminating the need to combine several detectors to measure polarisation, as done in many previous experiments and hence avoiding the consequent systematic effects. While the HWP is expected to suppress many systematic effects, some of them will remain. We use an analytical approach to comprehensively address the mitigation of these systematic effects and identify the characteristics of scanning strategies that are the most effective for implementing a variety of calibration strategies in the multi-dimensional space of common spacecraft scan parameters. We verify that <jats:italic>LiteBIRD</jats:italic>'s <jats:italic>standard configuration</jats:italic> yields good performance on the metrics we studied. We also present <jats:monospace>Falcons.jl</jats:monospace>, a fast spacecraft scanning simulator that we developed to investigate this scanning parameter space.</jats:p>