Use of close range photogrammetry in James Webb Space Telescope alignment testing under cryogenic conditions

Abstract

The James Webb Space Telescope (JWST) cryogenic testing required measurement systems that both obtain a very high degree of accuracy and can function in that environment. Close-range photogrammetry was identified as meeting those criteria. Extensive modeling prior to installation verified that the design would meet the desired accuracy goals. Extensive validation work was done to ensure that the actual as-built system met accuracy and repeatability requirements. The simulated image data predicted the uncertainty in measurement to be within specification and this prediction was borne out experimentally. Uncertainty at all levels of a measurement volume that comprised 8 × 8 m was verified experimentally to be <0.1 mm with a repeatability of <0.03 mm at the primary mirror (PM) level, achieving measurement accuracies on par with a laser tracker or radar system. During a 3-month Optical Telescope Element and Integrated Science Instrument Module thermal vacuum test performed in chamber A at Johnson Space Center, the photogrammetric system provided key data to allow for verification of actuator ranges needed for on-orbit alignment of JWST, alignment of the PM and secondary mirror (SM), and positional information needed for thermal model verification.