Fabrication of SiC aspheric mirrors with low mid-spatial error

Abstract

Recent experience with finishing off-axis parabolas and other conic surfaces is demonstrated by some examples that illustrate surface accuracy – not only in terms of traditional metrics, but also in terms of specified ranges of spatial frequency. Particular attention is given to the topic of interferometric metrology, and the extent to which we can effectively characterize mid-spatial frequency errors. The presence of mid-spatial errors can appear even more dominant in hard ceramics like SiC as compared with glass – reasons for this are suggested. This paper will discuss how controlled force grinding, robotic polishing, and surface smoothing can be employed to minimize and mitigate mid-spatial errors in fast silicon carbide aspheric mirrors. Recent experience and results are presented on two SiC mirrors finished by Aperture Optical Sciences Inc.