The research of nested grazing incidence x-ray telescope with high angular resolution

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.