The JWST infrared Scanning Shack Hartman System: a new in-process way to measure large mirrors during optical fabrication at Tinsley

Abstract

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 9.3um, 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. Although the scale of the SSHS is significantly larger than previous applications, substantial commercial technology is directly applicable to the Infrared SSHS with little or no modification. Extensive use was made of existing algorithms for frame stitching and focal spot centroiding. 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.

Optics technology for large-aperture space telescopes: from fabrication to final acceptance tests

“…13, where residual lathe turning marks are clearly visible in the x and y slope maps (a) and (b), respectively, but become dominated by the low-order shapes in the wavefront map (c). The machined aluminum surface from the lathe was too rough to obtain quality data, so the surface was ground with automotive body shop methods [149].…”

Design and characterization of a balloon-borne diffraction-limited submillimeter telescope platform for BLAST-TNG

“…Additionally, limited metrology data was available during the intermediate stages of the manufacture and for the final product. Traditional evaluation techniques for largeaperture infrared and submillimeter telescope optics such as infrared interferometry, 21 wavefront sensing, 24 and coordinate measuring machine (CMM) profilometry were all cost-prohibitive given the size of the primary mirror. Both the mold and the primary mirror surface were surveyed using a laser tracker, a technique which has been demonstrated at the <1 µm accuracy level, 25 and surface data recorded on a ∼5 cm square grid.…”

Shack–Hartmann centroid detection method based on high dynamic range imaging and normalization techniques

“…In our case, the philosophy behind the method differs from that in other proposed speckle reduction methods [13][14][15][16][17][18][19]. Instead of trying to eliminate the speckle noise in the SH CCD sensor, in our case, its presence is assumed and accepted, and it will be reduced using computer-based methods.…”

“…This problem typically arises in the cases of ocular wavefront sensing where a strong speckle field is formed by random interference of the coherent laser beam with the highly anisotropic tissues that form the retina [13][14][15][16][17]. Another case is the optical quality measurement of an instrument with rough surfaces [19].…”

“…Additionally, these devices can be used for wavefront reconstruction of coherent or incoherent optical beams. One of the main factors that restricts the performance of the SH sensor is the presence of a speckle field in a coherent incoming wavefront [13][14][15][16][17][18][19]. The nonlinear response and the restricted dynamic range imaging capabilities of the SH sensor reduce the accuracy (indeed the possibility) of the reconstructed wavefront.…”