Figure measurement of a large optical flat with a Fizeau interferometer and stitching technique

Large flat mirrors can be measured by using subaperture Fizeau interferometer and stitching the data. We have implemented such a system that can efficiently and accurately measure flat mirrors several meters in diameter using a 1 meter sub-aperture instantaneous Fizeau interferometer, coupled with sophisticated analysis software. The 1-m aperture optical system uses a fused silica test plate, reflective collimator, and commercial instantaneous interferometer. Collimator errors, mapping distortion, surface errors in the test plate, and other systematic effects were measured and compensated for individual measurements. Numerous individual maps were stitched together to determine the global shape of a 2-m class flat.