X-ray Optics Test and Calibration of the Einstein Probe Follow-up Telescope

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.