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Telescopes with very large diameter or with wide fields require convex secondary mirrors that may be many meters in diameter. The optical surfaces for these mirrors can be manufactured to the accuracy limited by the surface metrology. We have developed metrology systems that are specifically optimized for measuring very large convex aspheric surfaces. Large aperture vibration insensitive sub-aperture Fizeau interferometer combined with stitching software give high resolution surface measurements. The global shape is corroborated with a coordinate measuring machine based on the swing arm profilometer.
Burge et al. (Mon,) studied this question.