Full aperture super-smooth polishing and surface topology evolution of the mandrels for X-ray optics used in eXTP mission

The X-ray focusing mirror used in enhanced X-ray Timing and Polarimetry(eXTP) mission are manufactured by the electroformed nickel replication process, and the machining accuracy of the mandrels are directly related to the focusing quality of the mirror. The optical performance of the focusing mirror is not only affected by the shape error, but also closely related to the mid and high-spatial frequency errors. In this research, the full-aperture polishing process of NiP die was investigated, and the evolution of surface waviness and the surface roughness under subsequent polishing processes were examined. The 7 × 4 h of full-aperture polishing experiment was conducted on the self-developed two-axis polishing machine with optical grade pitch as the polishing tool. The experimental results showed that the mid-spatial frequency error of the mandrel was effectively reduced and the root mean square value of the surface roughness was decreased to 0.266 nm, which meets the stringent requirements for the surface quality of mandrel in eXTP mission.