<title>Rapid optical figuring of aspherical surfaces with plasma-assisted chemical etching</title>

Abstract

Rapid optical figuring of aspherical surfaces withPlasma Assisted Chemical Etching (PACE)L.D.Bollinger, C.Steinberg, C.B.ZarowinHughes Danbury Optical Systems100 Wooster Hts Rd, Danbury, CT 06810ABSTRACTProgrammed motion of a tool which removes material by Plasma Assisted ChemicalEtching (PACE) gives controlled, deterministic figuring. The process has beenautomated to correct arbitrary figure errors on aspheric surfaces directly from themeasured surface data. A system is now operating for figuring aspheric surfaces up to0.5 m diameter. PACE can greatly reduce the time and cost of figuring large optics byits rapid convergence to the final figure requiring only two to threemeasurement/figuring cycles to reach 1/50 wave surfaces. PACE intrinsically smoothshigh frequency roughness with material removal leaving a surface free of subsurfacedamage.1. INTRODUCTIONVery controllable, predictable material removal by Plasma Assisted ChemicalEtching (PACE) gives rapid convergence to the final surface figure for arbitrarilyshaped surfaces. For large, conventionally fabricated large optics, more than 50% ofthe fabrication time and cost (excluding the blank) results from the repeatedpolishing/metrology iterations needed to reach the final figure. The precise controlover material removal with PACE makes it possible to reduce the number of iterationsfrom 10-20 for conventional, mechanical polishing to 2-3 with PACE. In addition, therelatively high removal rates1, shown in Table 1, allows PACE to start earlier in thefabrication cycle at a level of figure generation. Experimentally measured andtheoretical analysis of PACE removal characteristics have been described in earlierpublications25. This paper will briefly review the concept of optical figuring byPACE, describe the PACE system constructed for large optic production, and discussexperimental results critical to rapid fabrication of large aspheric optics -