Optical designing of LiteBIRD

LiteBIRD aims to detect the footprint of the primordial gravitational wave on the Cosmic Microwave Background(CMB) in a form of polarization pattern called B mode. In order to separate CMB from the Galactic emission, our measurements cover 35 GHz to 450 GHz. The LiteBIRD optics consists of two telescopes: a crossed Dragone type for lower frequencies, which provides a compact configuration with a wide field of view, and a refractor type for higher frequencies. The whole optical system is cooled downto around 5 K to minimize the thermal emission. We use two kinds of approaches of designing calculations as well as the experimental confirmation particularly for the lower frequency telescope.