The Follow-up X-ray Telescope (FXT), a key payload onboard the Einstein Probe sallite (EP), is equipped with a Wolter-I x-ray focusing mirror system. We introduce the principle of such a mirror system and analyze the influence of the mirror gap in the multishell nested mirror of the FXT on the effective area, stray-light ratio, and vignetting. To ensure that no occlusion occurs within adjacent shells and minimize stray-light ratio, the size of the gap is set to a optimized value for corresponding shell. We finished a design of a 54-shell mirror system according to these results. The optical performance of the design was then simulated using a Monte Carlo algorithm and the ray-tracing principle. The simulation shows that the effective area is 414.5 ± 0.2 cm2 at 1.25 keV (considering the spider), and the field of view is 64 arcmin in diameter. These parameters meet the optical requirements of the FXT.
The Einstein Probe (EP) is an X-ray astronomical mission mainly devoting to time-domain astronomy. There are two main scientific payloads onboard EP, the Wide Field X-ray Telescope (WXT) based on the lobster eye optics and the Follow-up X-ray Telescope (FXT). FXT contains two Wolter-1 mirrors with a pnCCD detector on each focus. The total effective area is about 600 cm2 and the energy range is 0.3-10 keV. The pnCCD detector cooled by a pulse tube cooler enables high-resolution spectroscopy and imaging combined with excellent time resolution. It will also have several working modes with time resolution ranging from tens of microseconds to 50 milliseconds. Currently, the FXT is in its qualification model phase. The mirror assemblies (STM and TCM) as well as the pnCCD EM module have been manufactured and tested.
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