High-accuracy surface figure measurement of silicon mirrors at 80 K

Abstract

This report describes the equipment, experimental methods, and first results at a new facility for interferometric measurement of cryogenicallycooled spherical mirrors at the Goddard Space Flight Center Optics Branch. The procedure, using standard phase-shifting interferometry, has an standard combined uncertainty of 3.6 nm nns in its representation of the two-dimensional surface figure error at 80, and an uncertainty of rt 1 nm in the rms statistic itself. The first mirror tested was a concave spherical silicon foam-core mirror. with a clear aperture of 120 mm. The optic surface was measured at room temperature using standard "absolute" techniques; and then the change in surface figure error from room temperature to 80 K was measured. The mirror was cooled within a cryostat. and its surface figure error measured through a fused-silica window. The facility and techniques will be used to measure the surface figure error at 20K of prototype lightweight silicon carbide and CesicTM mirrors developed by Galileo Avionica (Italy) for the European Space Agency (ESA).