The optical design of the LiteBIRD Middle and High Frequency Telescope
  LiteBIRD is a JAXA strategic L-class mission devoted to the measurement of
polarization of the Cosmic Microwave Background, searching for the signature of
primordial gravitational waves in the B-modes pattern of the polarization. The
onboard instrumentation includes a Middle and High Frequency Telescope (MHFT),
based on a pair of cryogenically cooled refractive telescopes covering,
respectively, the 89-224 GHz and the 166-448 GHz bands. Given the high target
sensitivity and the careful systematics control needed to achieve the
scientific goals of the mission, optical modeling and characterization are
performed with the aim to capture most of the physical effects potentially
affecting the real performance of the two refractors. We describe the main
features of the MHFT, its design drivers and the major challenges in system
optimization and characterization. We provide the current status of the
development of the optical system and we describe the current plan of
activities related to optical performance simulation and validation.
