Simulation of the optical system in the GroundBIRD telescope.

GroundBIRD is a ground-based telescope for measuring the polarization of cosmic microwave background radiation, and it is soon to be operational at the Teide Observatory. The GroundBIRD telescope employs Mizuguchi-Dragone dual reflectors and 161 kinetic inductance detectors coupled with single polarization antennas as photon detectors. We present the results of our optical simulation on the pointing direction, stray light response, and influence of the blackbody radiation from the baffle. We also find that the power of the baffle radiation incident on the detectors is reduced by 99.95% when corrugated feed horns are coupled to the detectors.