Rapid fabrication strategy for empty set1.5 m off-axis parabolic parts using computer-controlled optical surfacing

Off-axis parabolic parts (OAPs) or quasi-OAPs are mostly frequently used in large optical telescopes. Compared to the stressed mirror polishing, computer-controlled optical surfacing (CCOS) or other computer-controlled subaperture tools provide more flexibility. However, the fabrication efficiency needs to be promoted in tactical ways. In this paper, we present a large aperture CCOS lap equipped with a compound motion unit and go through the grinding and pre-polishing with its figure errors. A CCOS-based heterocercal tool is first used in large optics to restrain the edge effects. In the fine polishing stage, corrective polishing, smoothing, and ion beam figuring are applied in combination to finish. We experimentally test this strategy on an empty set1.5 m OAP, as a part of giant steerable science mirror (GSSM) in the Thirty Meter Telescope. Finally, the surface error of empty set1.5 m OAP is better than 1/50 lambda RMS (full aperture), and the mid-spatial frequency part is better than 0.64 mu rad in slope RMS (effective aperture). The effective fabrication duration is reduced to 2 months. (C) 2018 Optical Society of America