Optical surface measurements for very large flat mirrors

Flat mirrors as large as 4 meters in diameter can be manufactured to high accuracy, limited by the surface metrology. We present metrology systems that are specifically optimized for measuring very large flats to high accuracy. A large aperture vibration insensitive Fizeau interferometer combined with stitching software provides high resolution surface measurements. Low order shape errors can be measured using scanning slope measurements from precision inclinometers or an autocollimator with scanning pentaprism.