Visible light blocking performance of a wide-field X-ray telescope onboard the Einstein probe

The wide-field X-ray telescope (WXT) onboard the Einstein probe (EP) is equipped with complementary metal-oxide semiconductor (CMOS) detector that is sensitive not only to X-rays but also to visible light. Therefore, effective visible light blocking is crucial to prevent interference with X-ray performance. In this paper, we present the design of the blocking films and the baffle for the WXT module and simulate the blocking performance on visible light. The visible light transmittance of the two film combinations is less than 10^-14 orders of magnitude. When the angle of incident light is beyond the field of view, the intensity is rapidly reduced by more than six times. We conducted experiments to test the out-of-field visible light blocking performance using a customized WXT module, and the measured results were in good agreement with the simulated results. On this basis, we calculated that when the EP satellite is operating in orbit, the reflected light from the Earth in the field of view would cause no significant response from the CMOS detectors. The on-orbit test results of the EP satellite verified our analysis, demonstrating that the WXT module has a good visible light blocking performance.