Aspheric Mirror Metrology for the HST Reflective Correctors

With the advent of advanced, automated fabrication techniques, the limiting factor in the production of complex aspheric space optics is increasing the ability to accurately measure and characterize optical surface parameters. In addition to increased accuracy, the metrology is required to keep pace with the shorter fabrication cycles of advanced figuring techniques such as computer-controlled polishing. As a result, there is an emphasis on: 1) Integrated testing approaches. Instruments such as interferometers must be interfaced with the polishing machine and the performance prediction software in order to minimize the time required to perform a full metrology cycle. 2) Redundancy in testing. Each surface parameter is measured in two independent, complementary test methods using unique test equipment. This minimizes the risk of systematic errors and reduces measurement uncertainty. 3) Overlapping measurement bandwidths. Test methods used to evaluate the surface errors are selected to ensure all spatial periods are adequately sampled and a complete characterization of the mirror surface is obtained.