NIST has accurately measured parts designed for the upcoming James Webb Space Telescope, the successor to the Hubble Space Telescope. NIST mechanical engineer John Stoup and his colleagues performed measurements of 4 alignment mounts that support and help to position a major Webb telescope structure known as the Primary Mirror Backplane Support Structure. Structure will hold telescope’s 21 ft diameter mirror as well as approximately 5,300 lb of telescope’s instruments and optical components.