Evaluation of the effective mirror area of CTA Small-Sized Telescopes for camera design and Monte Carlo simulation

Abstract: The effective mirror area of an imaging atmospheric Cherenkov telescope is a crucial key parameter for trigger threshold determination and energy calibration. It is usually calculated by 3D ray-tracing simulation using a simplified telescope model, and the result is used in Monte Carlo simulations. However, simplified telescope and camera models are not adequate for the Schwarzschild-Couder configuration to be used in Small-Sized Telescopes (SSTs) of the Cherenkov Telescope Array. This is because the complex 3D structure of the secondary mirror, telescope masts, and camera body block a significant fraction of Cherenkov and night-sky photons. To evaluate the effective mirror area of an SST and to finalize its camera body design with minimal shadowing, a complex 3D model was built and simulated using the ROBAST ray-tracing library. A camera body size of 570 mm and a window size of 430 mm were selected for the final camera design based on the evaluation of shadowing by simulation. A non-axisymmetric effective area distribution was determined via the modeling of the complex telescope structure, while meeting the SST effective area requirement.