JWST Optical Telescope Element Center of Curvature Test

The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) and Integrated Science Instrument Module (ISIM) completed their element level integration and test programs and were integrated to the next level of assembly called OTE/ISIM (OTIS) at Goddard Space Flight Center (GSFC) in Greenbelt, Maryland in 2016. Before shipping the OTIS to Johnson Space Center (JSC) for optical test at cryogenic temperature a series of vibration and acoustic tests were performed. To help ensure that the OTIS was ready to be shipped to JSC an optical center of curvature (CoC) test was performed to measure changes in the mirror's optical performance to verify that the telescope's primary mirror was not adversely impacted by the environmental testing and also help us in understanding potential anomalies identified during the JSC tests. The 6.5 meter diameter primary mirror consists of 18 individual hexagonal segments. Each segment is an off-axis asphere. There are a total of three prescriptions repeated six times each. As part of the CoC test each segment was individually measured using a high-speed interferometer (HSI) designed and built specifically for this test. This interferometer is capable of characterizing both static and dynamic characteristics of the mirrors. The latter capability was used, with the aid of a vibration stinger applying a low-level input force, to measure the dynamic characteristic changes of the PM backplane structure. This paper describes the CoC test setup and both static and dynamic test results.