Surface Figure Measurement at 20K: Silicon Carbide and Cesic Mirrors

This report presents the facility, methods, and results of testing cryogenically-cooled spherical mirrors, using standard phase-shifting interferometry, at the Goddard Space Flight Center (GSFC). Two mirrors were supplied to GSFC by the European Space Technology Center, so that GSFC could render a second, independent cryo-measurement of their surface figures at 20K. These mirrors, produced by Galileo Avionica and its partners, demonstrate the technology of silicon carbide and SiC-composite lightweighted mirrors, designed for high accuracy (10 nm rms surface figure error) at both room temperature and 20K. The two mirrors provided for testing at GSFC include one made of sintered silicon carbide (mirror blank by Bettini), and a second made of the C/Si/SiC composite known as Cesic (supplied by ECM). Both mirrors are f/2 spheres with 150 mm clear aperture, and with integral mounts. At GSFC, the mirrors will be measured first at at room temperature, making use of standard techniques, with a predicted uncertainty of 2 nm. Then each mirror will be tested in the cryostat, down to 20K: first freely supported and, second, mounted to a Cesic plate that models a Cesic optical bench. The uncertainty of the resultant surface figure error is predicted to be about 5 nm rms. Details of the uncertainty budget are discussed in a related poster. GSFC's techniques and results can be usefully compared to the measurements performed on the same two mirrors by and for Galileo Avionica, who used a somewhat different test configuration and alignment approach.