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LiteBIRD has been selected as JAXA's strategic large mission in the 2020s, toobserve the cosmic microwave background (CMB) $B$-mode polarization over thefull sky at large angular scales. The challenges of LiteBIRD are the widefield-of-view (FoV) and broadband capabilities of millimeter-wave polarizationmeasurements, which are derived from the system requirements. The possiblepaths of stray light increase with a wider FoV and the far sidelobe knowledgeof $-56$ dB is a challenging optical requirement. A crossed-Dragoneconfiguration was chosen for the low frequency telescope (LFT : 34--161 GHz),one of LiteBIRD's onboard telescopes. It has a wide field-of-view ($18^\circ\times 9^\circ$) with an aperture of 400 mm in diameter, corresponding to anangular resolution of about 30 arcminutes around 100 GHz. The focal ratio f/3.0and the crossing angle of the optical axes of 90$^\circ$ are chosen after anextensive study of the stray light. The primary and secondary reflectors haverectangular shapes with serrations to reduce the diffraction pattern from theedges of the mirrors. The reflectors and structure are made of aluminum toproportionally contract from warm down to the operating temperature at $5\,$K.A 1/4 scaled model of the LFT has been developed to validate the widefield-of-view design and to demonstrate the reduced far sidelobes. Apolarization modulation unit (PMU), realized with a half-wave plate (HWP) isplaced in front of the aperture stop, the entrance pupil of this system. Alarge focal plane with approximately 1000 AlMn TES detectors and frequencymultiplexing SQUID amplifiers is cooled to 100 mK. The lens and sinuousantennas have broadband capability. Performance specifications of the LFT andan outline of the proposed verification plan are presented.

Concept Design of Low Frequency Telescope for CMB B-mode Polarization satellite LiteBIRD