Yanji Yang

The Spectroscopy Focusing Array-Timing (SFA-T) constitutes a principal payload onboard China’s enhanced X-ray Timing and Polarimetry mission (eXTP). To achieve substantial effective area, optimize observational efficiency, and maintain high-precision positioning capabilities, the eXTP mission employs fifty nested Wolter-I mirror shells as its optical architecture across all payloads. Although payload-specific variations exist in coating selection and manufacturing specifications due to differing energy ranges and angular resolution requirements, the fundamental geometric design parameters remain consistent throughout the system. This study presents comprehensive optical simulations of the SFA-T system, which comprises five identical telescope modules. The optical configuration achieves an effective area of 910 cm ² at 1.50 keV with angular resolution characterized by HPD = 1 ′ and W90 = 3 ′ . Simulations reveal a field of view measuring 18. 4 ′ × 17. 2 ′ and demonstrate that stray light contributes merely 0.1% to central pixel contamination in multi-pixel Silicon Drift Detector, thereby eliminating the requirement for external X-ray baffles. Ground calibration measurements exhibit 29%–43% effective area reduction relative to on-orbit performance due to beam divergence effects. The integrated five-telescope configuration satisfies design specifications for effective area, thereby validating the optical performance for high-precision X-ray timing observations. Furthermore, all these simulation results will play an indispensable role in the calibration implementation and in establishing the on-orbit calibration database for the SFA-T.

The enhanced X-ray Timing and Polarimetry (eXTP) mission is China’s future flagship X-ray astronomy observatory. The eXTP payload comprises an array of Wolter-I focusing mirrors that utilize nickel electroforming technology. The testing and calibration of the eXTP mirror assemblies will be primarily conducted at the 100 m X-ray Test Facility (100XF) at the Institute of High Energy Physics, with support from the PANTER facility of the Max Planck Institute for Extraterrestrial Physics. Consequently, cross-calibration between the facilities is essential. This paper presents preliminary cross-calibration for mirror module 3 of the eXTP focusing mirror, conducted at both the 100XF and PANTER facilities, focusing on point spread functions (PSFs), vignetting, and effective area. The angular resolutions obtained at the two facilities are 21 . ″ 8 ± 0 . ″ 2@Al–K α , 29 . ″ 8 ± 0 . ″ 5@Cu–K α (100XF), as well as 21 . ″ 9 ± 0 . ″ 2@Al–K α , 27 . ″ 6 ± 0 . ″ 5@Cu–K α (PANTER). The measured effective area results at 100XF are 52.0 ± 0.7 cm ² @Al–K α , 12.1 ± 0.1 cm ² @Cu–K α . Meanwhile, after correcting for beam length (or divergence angle), the results from PANTER are 52.8 ± 0.7 cm ² @Al–K α , 12.2 ± 0.2 cm ² @Cu–K α . The cross-calibration results validate the reliability of the calibration methodologies for eXTP mirrors. Consistent outcomes in alignment, focus search, and PSFs were achieved, satisfying stringent mission requirements. Effective area measurements showed good agreement with simulations, with deviations limited to ±10%. These cross-calibration results provide valuable references for subsequent mirror calibrations and corrections, especially concerning effective area calibration, which should be a key focus of future research efforts.

The Einstein Probe (EP) mission, launched in 2024 January, is a time-domain X-ray satellite designed to discover and study X-ray transient and burst sources while investigating electromagnetic counterparts to gravitational waves. To achieve precise observations, EP features the Follow-up X-ray Telescope (FXT), equipped with two Wolter-I type mirrors for precise imaging within a 1 square degree field of view. A specialized Ray-Tracing package for the FXT (RTF) was developed to assess the Wolter-I type mirrors’ performance used in FXT. By employing specialized algorithms to simulate X-ray photon total reflection, RTF has yielded valuable insights into key optical parameters such as point-spread function, effective area, and vignetting. The package also evaluates components like the X-ray baffle and filter wheel, further enhancing the optical system’s efficacy. Moreover, the ray-tracing results have been validated against measurements. These simulations are crucial for ground calibration and will assist in maximizing the scientific output of the EP mission by improving the understanding of mirrors’ behavior in various observational conditions.

The X-ray satellite “Einstein Probe” of the Chinese Academy of Sciences (CAS) was successfully launched on 9 January 2024 at 15:03 Beijing Time from the Xichang Satellite Launch Center in China with a “Long March-2C” rocket. The Einstein Probe is equipped with two scientific X-ray telescopes. One is the Wide-field X-ray Telescope (WXT), which uses lobster-eye optics. The other is the Follow-up X-ray Telescope (FXT), a Wolter-I type telescope. These telescopes are designed to study the universe for high-energy X-rays associated with transient high-energy phenomena. The FXT consists of two modules based on 54 thin X-ray Wolter-I grazing incidence Ni-replicated mirrors produced by the Italian Media Lario company, as contributions from the European Space Agency and the Max Planck Institute for Extraterrestrial Physics (MPE), which also provided the focal-plane detectors. Meanwhile, the Institute of High Energy Physics (IHEP), together with the Harbin Institute of Technology and Xi’an Institute of Optics and Precision Mechanics, has also completed the development and production of the structural and thermal model (STM), qualification model (QM) and flight model (FM) of FXT mirrors for the Einstein Probe (EP) satellites for demonstration purposes. This paper introduces the precision manufacturing adopted in China of Wolter-I X-ray mirror mandrels similar to those used for the EP-FXT payload. Moreover, the adopted electroformed nickel replication process, based on a chemical nickel–phosphorus alloy, is reported. The final results show that the surface of the produced mandrels after demolding and the internal surface of the mirrors have been super polished to the roughness level better than 0.3 nm RMS and the surface accuracy is better than 0.2 μm, and the mirror angular resolution for single mirror shells may be as good as 17.3 arcsec HPD (Half Power Diameter), 198 arcsec W90 (90% Energy Width) @1.49 keV (Al-K line). These results demonstrate the reliability and advancement of the process. As the first efficient X-ray-focusing optics manufacturing chain established in China, we successfully developed the first focusing mirror prototype that could be used for future X-ray satellite payloads.