The Webb telescope features a mirror 21.3 feet (6.5 m) across, with nearly seven times the area of the mirror on the famed Hubble Space Telescope. In order to launch something this size, NASA designed the primary mirror to consist of 18 separate, hexagon-shaped mirrors that will fold up inside the rocket that carries the telescope into space. The mirror components are being tested both individually and together. A specially designed vacuum chamber at Marshall Space Flight Center simulates the extreme conditions of space, subjecting the mirror segments to temperatures as low as 400 degrees below zero.
The Incredible Expanding Telescope
Publication: February 23, 2010