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Proceedings Paper

High precision optical finishing of lightweight silicon carbide aspheric mirror
Author(s): John Kong; Kevin Young
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Paper Abstract

Critical to the deployment of large surveillance optics into the space environment is the generation of high quality optics. Traditionally, aluminum, glass and beryllium have been used; however, silicon carbide becomes of increasing interest and availability due to its high strength. With the hardness of silicon carbide being similar to diamond, traditional polishing methods suffer from slow material removal rates, difficulty in achieving the desired figure and inherent risk of causing catastrophic damage to the lightweight structure. Rather than increasing structural capacity and mass of the substrate, our proprietary sub-aperture aspheric surface forming technology offers higher material removal rates (comparable to that of Zerodur or Fused Silica), a deterministic approach to achieving the desired figure while minimizing contact area and the resulting load on the optical structure. The technology performed on computer-controlled machines with motion control software providing precise and quick convergence of surface figure, as demonstrated by optically finishing lightweight silicon carbide aspheres. At the same time, it also offers the advantage of ideal pitch finish of low surface micro-roughness and low mid-spatial frequency error. This method provides a solution applicable to all common silicon carbide substrate materials, including substrates with CVD silicon carbide cladding, offered by major silicon carbide material suppliers. This paper discusses a demonstration mirror we polished using this novel technology. The mirror is a lightweight silicon carbide substrate with CVD silicon carbide cladding. It is a convex hyperbolic secondary mirror with 104mm diameter and approximately 20 microns aspheric departure from best-fit sphere. The mirror has been finished with surface irregularity of better than 1/50 wave RMS @632.8 nm and surface micro-roughness of under 2 angstroms RMS. The technology has the potential to be scaled up for manufacturing capabilities of large silicon carbide optics due to its high material removal rate.

Paper Details

Date Published: 6 October 2010
PDF: 5 pages
Proc. SPIE 7655, 5th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Advanced Optical Manufacturing Technologies, 765503 (6 October 2010); doi: 10.1117/12.867923
Show Author Affiliations
John Kong, Precision Asphere, Inc. (United States)
Kevin Young, Precision Asphere, Inc. (United States)

Published in SPIE Proceedings Vol. 7655:
5th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Advanced Optical Manufacturing Technologies
Li Yang; Yoshiharu Namba; David D. Walker; Shengyi Li, Editor(s)

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