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The Follow-up X-ray telescope (FXT) is one of the instruments on board the Einstein Probe (EP) satellite of the Chinese Academy of Sciences (CAS) which was launched in January 2024. The EP mission is dedicated to the study of time-domain high-energy astrophysics, utilising a lobster-eye-based wide-field telescope, complemented by an eROSITA-like optics for follow-up observations. Max Planck Institute for Extraterrestrial Physics (MPE) has provided hardware and conducted measurement campaigns at its test facilities as part of a European contribution to Einstein Probe by ESA, and in addition the eROSITA flight spare mirror assembly as the second FXT module. Three FXT mirror assemblies – structural-thermal, qualification and flight models – have been manufactured. All components underwent acceptance testing using X-rays, followed by the installation of X-ray baffles for stray-light rejection. Subsequently, they underwent environmental tests and X-ray performance evaluations. The final tests of the qualification model, serving as a flight spare, and the flight model included an X-ray calibration at various photon energies ranging from about 0.3 keV to 8 keV. All tests were performed at MPE's test facilities: the laboratory for vibration and thermal-vacuum testing, and the PANTER X-ray facility. Reported are the setups and the results of the respective test sequences, focusing on the qualification and flight mirror assemblies. After delivery to China, all mirror assemblies were subjected to complementary measurements in the X-ray test facility of the Institute for High Energy Physics (IHEP) of CAS. • Design of the Follow-up X-ray telescope (FXT) on Einstein Probe. • FXT model philosophy and test matrix. • FXT X-ray test and calibration results. [ABSTRACT FROM AUTHOR]

Spectroscopy focusing array (SFA) and polarimetry focusing array (PFA) are the two major payloads of the enhanced X-ray Timing and Polarimetry mission (eXTP). Nested Wolter-I X-ray mirror module is implemented in SFA and PFA to achieve a high effective area. When evaluating the properties of the mirror module, the alignment of the optical axis of the X-ray mirror module and a quasi-parallel X-ray beam is a prerequisite for ensuring accuracy of the results. To assist the alignment of the X-ray mirror module, an X-ray focal plane detector with one of the largest detection areas, is designed based on a back-illuminated scientific complementary metal–oxide–semiconductor transistor (sCMOS) sensor(GSENSE6060BSI; Gpixel Inc). Then the characteristics of readout noise, dark current, and split-pixel event properties of the detector are studied with the self-developed multi-target fluorescence X-ray source in a 100-m long X-ray test facility. The energy calibration is performed based on the single-pixel event, and the energy non-linearity of the detector is also observed. Finally, based on the optical model, the eXTP mirror module is simulated, and the alignment test of the Wolter-I X-ray mirror module designed for Einstein Probe/Follow-up X-ray Telescope via the "Burkert test" method is demonstrated. [ABSTRACT FROM AUTHOR]