The center of curvature optical assembly for the JWST primary mirror cryogenic optical test: optical verification

Abstract

The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, allreflective, three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly, integration, alignment and testing requirements. A full aperture center of curvature optical test is performed in cryogenic vacuum conditions at the integrated observatory level to verify PM performance requirements. The Center of Curvature Optical Assembly (CoCOA), designed and being built by ITT satisfies the requirements for this test. The CoCOA contains a multi wave interferometer, patented reflective null lens, actuation for alignment, full in situ calibration capability, coarse and fine alignment sensing systems, as well as a system for monitoring changes in the PM to CoCOA distance. Two wave front calibration tests are utilized to verify the low and Mid/High spatial frequencies, overcoming the limitations of the standard null/hologram configuration in its ability to resolve mid and high spatial frequencies. This paper will introduce the systems level architecture and optical test layout for the CoCOA.

Pathfinder value for integration and test of the Webb Telescope optical system

“…7). 3 The interferometer was designed to measure the segmented separation of the primary mirror using three lasers. 4 Without access to the interferometer for 3 months during the flight hardware test, the lasers were moved outside of the chamber feeding light into optical fibers and a redundant camera was added inside the interferometer to dramatically increase the reliability.…”

James Webb Space Telescope (JWST) optical telescope element and integrated science instrument module (OTIS) cryogenic test program and results

“…Figure13. Fringe images from the COCOA interferometer for the aligned and phased JWST Primary Mirror.…”