A thermo-optical test device utilizing the 100-m x-ray test facility

X-ray mirror modules are the core components of X-ray astronomy research, which can focus X-rays from space and significantly improve detection sensitivity. This X-ray optical device are typically composed of nested multiple mirror shells and require maintaining a constant working temperature. Due to the thin-walled structure of the mirror shells and the fact that the inner surface reflects X-rays, direct contact temperature control is not feasible, making temperature control challenging. To evaluate the thermo-optical performance of the mirrors, based on the 100-m X-ray Test Facility (100XF) of the Institute of High Energy Physics (IHEP), a thermo-optical test device with high cleanliness was developed in this study. This system enables precise control of the mirror temperature and synchronous testing of X-ray performance, establishing the unique X-ray thermo-optical testing capability in China. The system consists of a high cleanliness level thermal sink, a liquid nitrogen circuit, multi-layer insulation, a temperature controller, and low-temperature probes. This system has demonstrated the capability to test the thermo-optical performance of X-ray mirror modules and has successfully conducted thermo-optical tests on the mirror module of the follow-up X-ray telescope (FXT) payload onboard the Einstein Probe (EP), achieving precise temperature control of the X-ray mirrors and testing its X-ray optical performance at different operating temperatures. The thermo-optical performance of the mirror module obtained from the thermal tests has been verified in-orbit. This paper provides a detailed description of the design, development, and validation of this system, as well as an overview of the results of the thermo-optical tests conducted on the FXT.