Photogrammetric Metrology for the James Webb Space Telescope Integrated Science Instrument Module
2007
The James Webb Space Telescope (JWST) is a 6.6m diameter, segmented, deployable telescope for cryogenic IR space
astronomy (~40K). The JWST Observatory architecture includes the Optical Telescope Element and the Integrated Science
Instrument Module (ISIM) element that contains four science instruments (SI) including a Guider. The ISIM structure must meet
its requirements at the ~40K cryogenic operating temperature.
The SIs are aligned to the structure's coordinate system under ambient, clean room conditions using laser tracker and theodolite
metrology. The ISIM structure is thermally cycled for stress relief and in order to measure temperature-induced mechanical,
structural changes. These ambient-to-cryogenic changes in the alignment of SI and OTE-related interfaces are an important
component in the JWST Observatory alignment plan and must be verified.
We report on the planning for and preliminary testing of a cryogenic metrology system for ISIM based on photogrammetry.
Photogrammetry is the measurement of the location of custom targets via triangulation using images obtained at a suite of digital
camera locations and orientations. We describe metrology system requirements, plans, and ambient photogrammetric
measurements of a mock-up of the ISIM structure to design targeting and obtain resolution estimates. We compare these
measurements with those taken from a well known ambient metrology system, namely, the Leica laser tracker system.
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