Subaperture Stitching Interferometry for Large Convex Aspheric Surfaces

The size and accuracy specifications of telescope mirrors are ever more demanding. This is particularly true for secondary mirrors, as they are convex and thus require large-aperture optics to test them. Recent NASA programs, such as the Terrestrial Planet Finder (TPF) and James Webb Space Telescope (JWST), include monolithic secondary mirrors of significant size (larger than half a meter). Secondary mirrors of such large sizes are difficult and expensive to test due to the large-aperture converging and nulling optics required. Furthermore, calibration of these optics to the level required for next-generation programs is extremely challenging. Subaperture stitching has the potential to provide accurate high-resolution maps of large-aperture aspheric optics without the use of even larger aperture optics or dedicated nulls. QED's Subaperture Stitching Interferometer (SSIREG) has achieved nanometer-level accuracies on spherical optics with its novel compensation techniques. Non-null capability is enhanced since the individual subapertures have significantly less aspheric departure. However, the system is currently only capable of testing up to 280 mm optics of mild asphericity. Phase I work demonstrated that the technology is scalable to larger aperture sizes. This proposal focuses on innovations for leveraging the considerable benefits of stitching (high resolution, automatic calibration, and flexible aspheric testing) to aspheric optics. Activities will include uncertainty analyses of aspheric tests, subscale measurement demonstrations, and development of concepts for testing larger amounts of aspheric departure.