An X-Ray Optic System Ray-tracing Package for FXT on Board EP Satellite

Abstract The Einstein Probe (EP) mission, launched in 2024 January, is a time-domain X-ray satellite designed to discover and study X-ray transient and burst sources while investigating electromagnetic counterparts to gravitational waves. To achieve precise observations, EP features the Follow-up X-ray Telescope (FXT), equipped with two Wolter-I type mirrors for precise imaging within a 1 square degree field of view. A specialized Ray-Tracing package for the FXT (RTF) was developed to assess the Wolter-I type mirrors’ performance used in FXT. By employing specialized algorithms to simulate X-ray photon total reflection, RTF has yielded valuable insights into key optical parameters such as point-spread function, effective area, and vignetting. The package also evaluates components like the X-ray baffle and filter wheel, further enhancing the optical system’s efficacy. Moreover, the ray-tracing results have been validated against measurements. These simulations are crucial for ground calibration and will assist in maximizing the scientific output of the EP mission by improving the understanding of mirrors’ behavior in various observational conditions.