Filter transmittance calibration of the follow-up x-ray telescope on board the EP satellite

The Einstein Probe satellite is equipped with two payloads, one of which is the Follow-up X-ray Telescope (FXT). The FXT comprises two completely independent optical and detector systems: FXT-A and FXT-B. Each system is equipped with a rotating wheel that has six positions, which include three types of optical filters: thin filter, medium filter, and hole position. Filters made from different materials and thicknesses can be adjusted under various conditions to optimize the detector’s photon reception. The primary observation energy range of the FXT is 0.3–10 keV. This article primarily focuses on determining the transmittance of different optical filters at various energy levels. This includes the transmittance of thin filters, medium filters, and hole positions corresponding to 14 energy points of FXT-A, as well as the transmittance of filters at 10 energy points of FXT-B. In addition, based on the results of the experiment, simulations were conducted on the variation of transmittance with energy, and the actual thickness of the filters was obtained.