X-ray focusing optics is one of the most important technologies for X-ray astronomy with capabilities of direct imaging, low background and high sensitivity. The enhanced X-ray Timing and Polarimetry (eXTP) and hot universe Baryon surveyor (HUBS), are proposed in China as two major X-ray missions for the next decade. The thermally slumping glass optics (SGO) is one of the approaches to satisfy the requirement of large collecting areas and high angular resolution. In the last years, we made a great progress in the research of SGO X-ray focusing telescope and established the whole fabrication platform. This paper will give a review of recent developments in each step, including lens fabrication, multilayer coating, telescope assembly and optical performance testing. We established an in-situ multi-probe platform based on the contact, trigger and non-contact probes. Thus, the radius deviation and the straightness of graphite strip, the surface shape and the radius of the inner surface of each layer can be measured during the telescope assembly process. We also developed a three-dimensional ray-tracing method to calculate the performance degradation induced by the deformed surface appearing on the forming process and optical assembly. Then the result could be feed back into the fabrication process for improving upon the later mounted mirrors in the real time. We have successfully fabricated two telescope prototypes with the focal length of 2000mm in the last years. The optical performance of the two 3-layer and 21-layer prototypes, which was numbered as No.6 and No.7 prototype respectively, were tested in PANTER test facility of MPE in September 2018. The measured angular resolution t of section A of No.6 prototype is 67±2 arc-second at 8keV, and the measured angular resolution in full aperture of No.7 telescope prototype is 96±3 arc-second at 8keV, which is much better than the previous prototypes.
Several imaging x-ray telescope (IXT) prototypes have been fabricated independently by the Institute of Precision Optical Engineering, which employed thermal slumping technology. To verify the performance of the IXT prototypes, a three-layer prototype with a focal length of 2052.5 mm was tested using a narrow beam at the Shanghai Synchrotron Radiation Facility. The performance testing posed a challenge due to the need to suppress the finite source distance effect on the IXT prototype (43-m long source-optic distance). In addition, limited use of motorized stages presents challenges. We present the experimental setups and detailed measurement approaches by utilizing limited measurement devices. The prototype is a segmented telescope comprising six sectors. For the best sector, the measured point spread function (PSF) yields a half power diameter (HPD) of 66″ and agrees well with modeling (62″) and the value measured at PANTER (65″). In addition, the integrated HPD of the whole prototype is 82″ obtained by coadding the PSFs of the six sectors.
The enhanced X-ray Timing and Polarimetry (eXTP) is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. One of the approaches to obtain large collecting areas is mounting the thermally slumping glass segments (SG) in a nested conical approximation Wolter-I design telescope. In the last two years, we have made a great progress in the research of telescope on the base of thermally slumping glass technology, and successfully fabricated several prototypes for the eXTP SFA mission. This paper intends to provide an overview of the progress. Now we can routinely produce cylindrical glasses with 36-60″ resolution (HPD) and the best mirror produced has an angular resolution of 36″ (HPD). The glass substrates coated with Pt and C layers to obtain high reflectivity of X-ray at 1-10keV, and the reliability of multilayers coating studies are under way as well. The in-situ measurement system and 3-dimention ray-tracing program have developed and can feed measurement data back into the assembly process for improving upon the later mounted mirrors in real time. We fabricated a 3-layer telescope prototype with the diameter of 106mm and focal length of 2000mm, and tested the focusing performance in SSRF, China. The measured HPD is 66″, and W90 is 168″, which meets the requirement of angular resolution of eXTP/SFA.