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 -60 C using a 10.6 μm 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 μm over the 80 C temperature change. The effective coefficient of thermal expansion over this temperature range derived from the focus change is 0.66 x 10-6/C, close to that of fused silica.

We have been able to utilize silicon monoxide (SiO) as a refractory coating to improve on the replicated figure of composite optics made from carbon fiber reinforced plastic (CFRP) sandwich panels. We apply an evaporative coating of SiO to the surface of the mirror and polish that coating to obtain the desired figure for the complete mirror. We have developed this technique to allow the use of CFRP optics which are both stable and have a low areal density. This represents a novel use of thin film vacuum coating to produce an optical substrate rather than a simple reflective or protective overcoating and represents an advance in producing thick SiO coatings (10 - 40 micrometers) over large areas (25 - 60 cm).

We have been able to utilize silicon monoxide (SiO) as a refractory coating to improve on the replicated figure of composite optics made from Carbon Fiber Reinforced Plastic (CFRP) sandwich panels. We apply an evaporative coating of SiO to the surface of the mirror and polish that coating to obtain the desired figure for the complete mirror. We have developed this technique to allow the use of CFRP optics which are both stable and have a low areal density. This represents a novel use of thin Nm vacuum coating to produce an optical substrate rather than a simple reflective or protective overcoating and represents an advance in producing thick SiO coatings (10-40 mu m) over large areas (25-60 cm).