Optical testing has been completed on a replica of NASA's James Webb Space Telescope (JWST) while construction of the real McCoy reached a critical milestone. Engineers are testing nonflight equipment on a mockup called Pathfinder, which copies the Webb telescope's center section backplane. Engineers inspect the Pathfinder telescope after its second super-cold optical test at NASA's Johnson Space Center in Houston. Images courtesy of Chris Gunn/NASA. "Practice makes perfect," said Mark Clampin, Webb telescope observatory project scientist at NASA's Goddard Space Flight Center. "Since we will be testing the world's largest-ever cryogenic telescope for the first time in the world's largest cryogenic test chamber, we need to be experienced in using our test equipment so we can focus on the performance of the telescope." After the first Pathfinder test was completed in June, the aft optics system (AOS), which includes the tertiary mirror and fine steering mirror, was installed on the Pathfinder to prepare for a second test. Fiber-fed IR optical sources were used to simulate star images, while an IR detector was used to perform end-to-end testing of the full Pathfinder telescope system. The AOS and the source system were built by Ball Aerospace and Technologies Corp.'s facilities in Boulder, Colo. Meanwhile, the first of 18 flight mirrors was installed in the JWST using a robot arm to lift and lower the hexagonal segment, which measures 4.2 ft and weighs approximately 88 lb. After being placed together, the 18 primary mirror segments will work together as one 21.3-ft mirror. An engineer installs the first flight mirror onto the James Webb Space Telescope at NASA's Goddard Space Flight Center in Greenbelt, Md. The mirrors are made of ultralightweight beryllium, chosen for its thermal and mechanical properties at cryogenic temperatures. Each segment also has a thin gold coating chosen for its ability to reflect IR light. The telescope's biggest feature is a tennis-court-sized five-layer sunshield that attenuates heat from the sun more than a million times. "After a tremendous amount of work by an incredibly dedicated team across the country, it is very exciting to start the primary mirror segment installation process," said Lee Feinberg, JWST optical telescope element manager at Goddard. "This starts the final assembly phase of the telescope." The mirrors must remain precisely aligned in space in order for the telescope to successfully carry out science investigations. While operating at extraordinarily cold temperatures between −406 and −343 °F, the backplane must not move more than 38 nm. The full installation is expected to be complete in early 2016. "The [JWST] will be the premier astronomical observatory of the next decade," said John Grunsfeld, astronaut and associate administrator of the Science Mission Directorate at NASA Headquarters in Washington. "This first-mirror installation milestone symbolizes all the new and specialized technology that was developed to enable the observatory to study the first stars and galaxies, examine the formation stellar systems and planetary formation, provide answers to the evolution of our own solar system, and make the next big steps in the search for life beyond Earth on exoplanets." The JWST is the scientific successor to NASA's Hubble Space Telescope, and will be the most powerful space telescope ever built. It will study every phase in the history of our universe, including the cosmos' first luminous glows, the formation of solar systems capable of supporting life on planets like Earth, and the evolution of our own solar system. JWST is an international project led by NASA in partnership with the European Space Agency and the Canadian Space Agency.