Precision polishing of the mandrel for X-ray grazing incidence mirrors in the mission of Einstein Probe

Based on chemical-mechanical polishing and combining mechanical and tribochemical polishing techniques, a precision lapping method suitable for parabolic polishing specified by grazing incident X-rays is introduced. Various factors affecting the polishing process are analyzed, and the results show that the proposed method is consistent with Preston's equation and Hertz contact principle. Therefore, this paper proposes a general material removal model based on the above two methods. The mid and high-spatial frequency errors are demanded to reach the requirements with an angular resolution consistently < 6 arcsec HEW and a roughness of 0.3 nm rms (between 1 mm and 0.002 mm spatial frequency range). To achieve the conformal ultra-smooth polishing of focusing mirrors, the process of full-aperture super-smooth pitch polishing is investigated. The influences of key polishing parameters are revealed. The evolution of the surface topology has been studied. A polishing setup is established to carry out experimental polishing to verify the optimum processing parameters obtained by simulations and previous polishing tests. Besides, the effect of abrasive particle size on the roughness is also verified. The roughness of the polished mandrel is measured at different positions, and the optimum roughness reaches Ra 0.359 nm. The polishing approach can significantly reduce the surface roughness of the replication mandrel, satisfying the low energy band focusing requirement of grazing incidence X-ray mirrors.