The James Webb Space Telescope's (JWST) end-to-end optical system will be tested in a cryogenic vacuum environment before launch at NASA Johnson Space Center’s (JSC) Apollo-era, historic Chamber A thermal vacuum facility. During recent pre-test runs with a prototype “Pathfinder” telescope, the vibration in this environment was found to be challenging for the baseline test approach, which uses phase retrieval of images created by three sub-apertures of the telescope. To address the vibration, an alternate strategy implemented using classic Hartmann test principles combined with precise mirror mechanisms to provide a testing approach that is insensitive to the dynamics environment of the chamber. The measurements and sensitivities of the Hartmann approach are similar to those using phase retrieval over the original sparse aperture test. The Hartmann test concepts have been implemented on the JWST Test Bed Telescope, which provided the rationale and empirical evidence indicating that this Hartmann style approach would be valuable in supplementing the baseline test approach. This paper presents a Hartmann approach implemented during the recent Pathfinder test along with the test approach that is currently being considered for the full optical system test of JWST. Comparisons are made between the baseline phase retrieval approach and the Hartmann approach in addition to demonstrating how the two test methodologies support each other to reduce risk during the JWST full optical system test.

Date Published: 29 July 2016
PDF: 12 pages
Proc. SPIE 9904, Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave, 99040C (29 July 2016); doi: 10.1117/12.2233114

Published in SPIE Proceedings Vol. 9904:
Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave
Howard A. MacEwen; Giovanni G. Fazio; Makenzie Lystrup; Natalie Batalha; Nicholas Siegler; Edward C. Tong, Editor(s)