Polishing an off-axis aspheric mirror by ion beam figuring

Abstract

Base on the special characters of off-axis spherical and three-axis ion beam figuring (IBF) system, a method of obtaining the removal function depending on incidence angles are introduced and the curve be indicated. Keeping the fabrication coordinate coincident to the optical coordinate can avoid the transformation of interferometry map, which could introduce the distortion error. By analyzing of the process of polishing the 562mm×290mm off-axis mirror, the polishing experimentation has been carried out on the IBF2000 system developed by NTG. After two iterations and 37.2 hours polishing time, the RMS value of surface accuracy is from 0.033λ to 0.016λ, and the mid-spatial error mitigated dramatically. The experimentation indicates that using the non-contacting polishing feature of IBF with reasonable optimizing process, the surface accuracy and the mid-spatial error can be improved simultaneously.