The eXTP (enhanced X-Ray Timing and Polarimetry) mission is a Chinese science space mission developed in collaboration with many international countries. Devoted to observations in the X-ray band, with imaging, spectroscopic, timing and polarimetry capabilities, is now entering phase B. The payload includes 9 Spectroscopic Focusing Array (SFA) and 4 Polarimetry Focusing Array (PFA) telescopes. The SFA telescopes, equipped with SDDs, have a spatial resolution of 1 arcmin, while the PFA telescopes, equipped with imaging gas pixel photoelectric polarimeters, have a spatial resolution of 30 arcsec. Both optics work in the 0.5-10 keV range with a focal length of 5.25 m and a field of view of 12 arcmin. The technology used for the optics production is Nickel electroforming from super-polished mandrels, like many previous successful X-ray missions. The reflecting coating is a double layer Au+C, which ensures optimal response at high and low energies. The PFA and SFA have the same optical design, in order to minimize the number of mandrels to be produced. In this paper, we present the optical design of these telescopes assisted with raytracing and a preliminary concept for the mechanical design supported by FEM simulation.
Electroless nickel is an excellent material for making optical molding dies, because of its machinability with both single-point diamond turning and polishing, as well as its suitable hardness and durability. This paper deals with the ultra-precision polishing of molding dies for telescope mirrors with a super smooth surface with a roughness below 1 nm root-mean- square (rms). According to the probability density distribution function of different sizes of abrasive grains cut into the workpiece, a surface roughness model combining the micro contact model and the force balance principle was established. A surface roughness of 0.316 nm Ra was obtained on slab molding dies of 30 mm in diameter.
The Medium Energy X-ray Telescope (ME), covering 5-30 keV, is one of the three main payloads of the Hard X-ray Modulation Telescope (HXMT). ME adopts an array of Si-PIN detectors. The detection area of one pixel is 56.25 mm<sup>2</sup> , and the total detection area is 952 cm<sup>2</sup> . The ME has a large active area while the pixel size is smaller. So the front-end electronics and forming electronics are realized by Application Specific Integrated Circuit (ASIC) chips. In this paper, we will describe the matching design of ME Si-PIN detector and ASIC, and the performance of the design. The energy response, temperature response, and dead time of a Two-Pixels Si-PIN detectors with the simplest readout electronics which is similar with ME, were tested on the Max Planck Institute for Extraterrestrial Physics PANTER X-ray test facility at Neuried by Munchen (Germany). The overall performance is quite similar to what was expected.
As China’s first X-ray astronomical satellite, Insight-HXMT (Hard X-ray Modulation Telescope) successfully launched on Jun 15, 2017. It performs timing and spectral studies of bright sources to determine their physical parameters. HXMT carries three main payloads onboard: the High Energy X-ray telescope (HE, 20-250 keV, NaI(Tl)/CsI(Na)), the Medium Energy X-ray Telescope (ME, 5-30 keV, Si-Pin) and the Low Energy X-ray telescope (LE, 1-15 keV, SCD). In orbit, we have used the radioactive sources, activated lines, the fluorescence line, and Cas A to calibrate the gain and energy resolution of the payloads. The Crab pulsar was adopted as the primary effective area calibrator and an empirical function was found to modify the simulated effective areas. The absolute timing accuracy of HXMT is about 100us from the TOA of Crab Pulsar.