The James Webb Space Telescope (JWST) primary mirror is 6.6 m in diameter and consists of 18 hexagonal mirror segments each approximately 1.5 m point-to-point. Each primary mirror segment assembly (PMSA) is constructed from a lightweight beryllium substrate with both a radius-of-curvature actuation system and a six degree-of-freedom hexapod actuation system. With the JWST being a near to mid-infrared observatory, the nominal operational temperature of a
PMSA is 45 K. Each PMSA must be optically tested at 45 K twice, first to measure the change in the surface figure & radius-of-curvature between ambient & cryogenic temperatures and then to verify performance at cryo following final polishing. This testing is conducted at Marshall Space Flight Center's (MSFC's) X-Ray & Cryogenic Facility (XRCF). The chamber & metrology system can accommodate up to six PMSAs per cryo test. This paper will describe the optical metrology system used during PMSA cryogenic testing. This system evolved from systems used during the JWST mirror technology development program. The main components include a high-speed interferometer, a computer-generated holographic null, an absolute distance meter, a tiltable window, and an imaging system for alignment. The optical metrology system is used to measure surface figure error, radius-of-curvature, conic constant, prescription
alignment, clear aperture, and the range & resolution of the PMSA actuation systems.