Self-referencing, motion-insensitive approach for absolute aspheric profiling of large optics to the nanometer level and beyond

We describe a new noncontacting approach for obtaining the full aperture, absolute aspheric profile of large optical surfaces. The metrology instrument is placed in close proximity to the test piece instead of at the center of curvature, and is thus equally useful for measuring concave, flat, and convex optics - even fast (low f-number) optics. It combines the data from multiple probes in a manner that makes the measurement completely self-referencing, and completely insensitive to any small relative rigid body motions between the instrument and the test piece. The relative compactness of the instrument combined with its inherent rigid body insensitivity make it suitable ultimately for in situ measurements. Furthermore, replacement of the noncontacting optical probes with contacting mechanical probes would make the instrument suitable for profiling ground surfaces to a very small fraction of a micron. We have built a prototype instrument to prove the concept, and have demonstrated sub-nanometer capabilities for the optical probes, with full surface figure accuracy capabilities of a few nanometers in an uncontrolled thermal environment. The full surface figure accuracy is improving as we implement modest environmental controls. In this paper, we first describe the underlying theory of the measurement approach, and then describe the prototype instrument. Finally, we summarize the measurements made to date, and discuss likely future applications and projected accuracies.