Tinsley, under JWST funding, has led the team that has developed a novel and highly versatile piece of ground support equipment for optical surface testing of JWST beryllium mirror segments during optical fabrication. The infrared Scanning Shack Hartmann System (SSHS) offers the advantage of being able to characterize mid-to-high spatial frequency structure on a mirror from early stages of fabrication when slopes may be high and surface irregular, eliminating the need for an extra polishing step before metrology. Working at 9.3μm, the system will accept and measure a wide dynamic range of surface characteristics, including roll-off near the edge of the segment. Knowledge of these surface features at the early grinding stage is imperative if characteristics such as mirror edge roll-off are to be minimized. WaveFront Sciences, producer of commercial COAS and Columbus Shack Hartmann systems, has provided systems engineering and component support for the SSHS system.
The SSHS system is based around a special Long Wave Infrared (LWIR) wavefront sensor developed by WaveFront Sciences that is scanned over the mirror surface, making sub-aperture measurements. The smaller, high-resolution measurements are then stitched together to provide high-resolution measurement of the entire mirror surface, even though the surface is in a rough ground state.
The system leverages technology from smaller visible instrumentation produced by Wavefront Sciences, especially those for surface sub-aperture measurements of semiconductor wafers. This paper will describe the implementation of the first infrared scanning Shack Hartmann system at Tinsley to address optical fabrication optimization of the JWST Primary Mirror Segments.
The context, preparation, and facilitization of Tinsley to produce the 18 JWST primary mirror segments are described,
and an overview of the Project at Tinsley is presented. The mirror segments are aggressively lightweighted,
approximately hexagonal, and approximately 1.32m flat-to-flat. While the optical finishing approach is strongly seated
in Tinsley's Computer Controlled Optical Surfacing (CCOSTM) technology, extensions have been implemented to
address safe and efficient nearly simultaneous flow of the high value mirror segments through numerous cycles of
optical finishing, processing and metrology steps. JWST will operate at cryogenic temperatures, and Tinsley will do
final figuring from a "hit map" made during cryogenic testing at the NASA MSFC X-Ray Calibration Facility (XRCF).
A formal beryllium safety protocol has been established throughout. Extensive handling fixtures assure that the mirrors
are moved from station to station experiencing low accelerations. A rigorous qualification process is applied to each
new fixture, machine and instrument. Special problems of cryo figuring, and co-finishing the segments to stringent
specifications are described.
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