Cryogenic optical performance of a lightweighted 20-inch SiC mirror and indications for thermal strain homogeneity and hysterisis

The properties of silicon carbide (low CTE, high modulus, high conductivity, low density) are ideal for mirrors performing at cryogenic temperatures. Test data at cryogenic temperatures indicate high thermal strain homogeneity as well as low hysteresis (critical properties for high quality optical performance). Until recently, the largest SiC mirrors tested at liquid helium temperatures have been only a few centimeters in diameter. Recently a lightweighted (6 kg) 20-inch-diameter SiC mirror manufactured by United Technologies Optical Systems was tested for figure change at 10 K. Hysteresis was quantified upon return to room temperature. The results indicate high thermal strain homogeneity and low hysteresis. These optical results are applied to a parametric model developed from numerous previous cryogenic tests to estimate the thermal strain variability. Quantitative comparisons to other cryogenic materials are made based on reported test data.