Development of Wolter I X-ray focusing mirror using the nickel replica in IHEP

The Einstein Probe (EP) mission, approved by the Chinese Academy of Sciences (CAS) in 2017 and launched in 2024, is a significant advancement in time-domain astronomy. The Follow-up X-ray Telescope (FXT), equipped with a Wolter I focusing mirror system and a PNCCD detector, is critical for detecting and analyzing cosmic X-rays. To ensure its reliability in space, structural and structural-thermal models were developed, and mechanical and thermal tests were conducted to verify its ability to withstand launch stresses and space conditions. The Wolter I mirrors, manufactured and integrated by the HIT team, achieved an angular resolution of 57 arcseconds at 1.5 keV and an effective area of 250 cm². This paper presents the verification results of these tests, including insights from the flight spare model, and discusses potential reasons for the observed loss of effective area, such as alignment imperfections or thermal distortions. These findings are essential for optimizing the FXT’s design and ensuring the mission’s success, highlighting the importance of rigorous testing in space-based observatory development.