Fabrication and optical characterization of a segmented and brazed mirror assembly

Direct Sintered Silicon Carbide (SSiC) is a promising material to fabricate large (over 1 meter diameter) land and space based mirror optics due to its low areal density, high stiffness and high thermal stability. To make large mirror optics for visible wavelength applications, sub-nanometer surface roughness is required, which can be achieved by cladding a SSiC substrate using SiC chemical vapor deposition (CVD). Limitations on available equipment to clad monolithic structures of this size require that smaller segments need to be clad first and then joined prior to being optically finished. To demonstrate the viability of this method of fabrication, a segmented n300mm visible quality lightweighted concave mirror has been manufactured and characterized. The mirror's 6 radial segments, coated with a SiC CVD layer on the SSiC substrate were joined by means of a silicon based braze, formulated so that its thermal expansion matched that of the SSiC substrate and SiC CVD layer. After figuring and polishing to optical quality, the mirror's stability was characterized under vacuum at three temperatures (120 K, 293 K, and 520 K) by measuring the wave front error (WFE).