Cryogenic testing under way on Webb telescope mirrors

  • 28-Jan-2010 08:50 EST

The James Webb Space Telescope’s primary mirror is made up of 18 hexagonal-shaped mirror segments. Each segment weighs 88 lb and measures 4.3 ft across. Ball Aerospace

One of the goals of the James Webb Space Telescope (JWST)—expected to launch in 2014—is to give scientists clues about the formation of the universe and the evolution of our solar system by observing galaxies more than 13 billion light years away. To be able to view such far-off and faint objects, a large mirror is needed to collect light from objects being observed.

NASA is developing a primary mirror, comprised of 18 hexagonal mirror segments, that is 21.6 ft in diameter—almost three times as large as the Hubble Space Telescope’s mirror. To ensure that the mirror segments can withstand the extreme space environment, principal optical contractor Ball Aerospace & Technologies Corp. in January shipped the first six JWST mirror segments to the X-ray and Cryogenic Facility (XRCF) at NASA’s Marshall Space Flight Center in Huntsville, AL, to undergo cryogenic temperature testing.

"By the time testing in the XRCF concludes in 2011, all 18 flight segments will have been through multiple measurements while experiencing the extreme temperatures of space," said Helen J. Cole, JWST Activities Project Manager at NASA Marshall.

The first XRCF test measures distortion of each mirror surface as it cools from room temperature to the Webb telescope's on-orbit operating temperature of approximately -380°F. This surface distortion is mapped and subsequently removed in the final mirror polishing operations. The second XRCF test performed on each mirror will verify that the warm-to-cold surface distortion has been properly removed in final polishing.

The XRCF is considered to be the world's largest X-ray telescope test facility and a unique, cryogenic, clean room optical test location. The test chamber takes approximately five days to cool a mirror segment to cryogenic temperatures.

"This is a tremendously important milestone to the Webb Telescope project that bodes well for both our future mirror manufacturing schedule and for the potential performance capabilities of the telescope," said Lee Feinberg, JWST Optical Telescope Element Manager at NASA Goddard.

When the primary mirror is assembled in space, it will include three different shapes of mirror segments: six A, six B, and six C segments. The testing will collect data from all three sizes. The engineering development unit, the first primary mirror segment of the Webb telescope that has met flight specifications at ambient temperatures, is also being tested.

Ball Aerospace began manufacturing the mirrors six years ago. Brush Wellman made 21 500-lb hexagonal mirror blanks from beryllium, an extremely strong, lightweight metal. Axsys Technologies machined the back side of the blanks and chemically etched them into an isogrid pattern that reduced mirror mass by 92%. The front side of the mirror blank was machined to prep the optical surface for high-precision grinding, polishing, and testing, which is being done by L-3 SSG-Tinsley. The JWST program is being led by prime contractor Northrop Grumman Corp.

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