Polishing and testing of the 3.5 m SiC M1 mirror of the Herschel space observatory of ESA

摘要

Herschel space observatory of ESA has a parabolic M1 mirror of silicon carbide (SiC) with large diameter of 3.5 m, fast focal ratio of f/0.5 and extreme light-weighting to 25 kg/m~(2), which make the polishing of the mirror a very challenging task. Use of very high surface pressure is necessary to polish efficiently this hard material, which increases the risk of quilting effects to the shape of the only 2.5 mm thick front face of the mirror. In this paper we present descriptions of the testing and polishing methods used during Herschel M1 lapping and polishing to the specified surface shape accuracy < 1.5 (mu)m rms and micro roughness <30 nm rms.