PHAST: plano holographic aspheric stitching technique

Telescope design advancements are leading to the need for larger convex secondary elements, making the use of traditional refractive test geometries impractical. In response to requests for larger convex components, Harris has developed the Plano Holographic Aspheric Stitching Technique (PHAST)^[1], a novel metrology approach that offers versatility as well as improved performance for large convex components. This approach was conceived initially for the in-process testing of the Large Synoptic Survey Telescope (LSST) M2^[2], a 3.4-meter diameter convex asphere and has since been expanded to a versatile design that can be quickly modified to test multiple prescriptions with minimal cost and schedule impacts. The metrology system has facilitated the production of the largest convex optic that Harris has processed and tested. The metrology approach is a sub-aperture stitching technique that uses a diffractive nulling element. This leverages the rapid production times of the lithography industry to reduce the lead time for test set assembly. For the most common convex component geometries, this test can be ready for use in as little as six months from receipt of specifications. We will present the development and design of this test methodology. Existing PHAST systems are providing high resolution and accurate data while demonstrating the stability of the overall approach. In addition, the approach is capable of rapid reconfiguration to accommodate testing of multiple convex optics over a range of sizes and specifications.