Low-temperature figure stability of a COI all-composite mirror M4

摘要

Abstract: Recent developments in the design and fabrication of very light-weight all-composite mirrors have made possible extremely well balanced, thermally stable, structures which distort very little when cooled. One such mirror is the Composite Optics, Incorporated all-composite mirror, M4, which has a 45.7 cm diameter and 3 cm thickness and a spherical surface of radius-of-curvature 2.92 meters. Relative figure measurements of this mirror were made with the Steward Observatory Light Weight Mirror Low Temperature Test Chamber over a temperature range from 20 C to $MIN@60 C using a 10.6 micrometer interferometer. The measurements show a remarkably small increase in the rms figure departure from a spherical surface of fixed radius-of-curvature of 0.27 micrometer over the 80 C temperature change. The effective coefficient of thermal expansion over this temperature range derived from the focus change is 0.66 $MUL 10$+$MIN@6$//C, close to that of fused silica. !4