JWST Mirror Segment Meets Cryogenic Specifications

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The first primary mirror segment of NASA’s James Webb Space Telescope (JWST) has been polished to its required specifications as measured at operational cryogenic temperatures, setting the stage for polishing of the telescope’s remaining 18 flight-mirror segments in preparation for launch in June 2014, NASA said in a March 2 press release.

“Many predicted it would take us multiple iterations to successfully polish these mirror segments to achieve the correct optical prescription at the telescope’s operating temperatures, but we did it on our first try,” said Scott Willoughby, JWST program manager at Redondo Beach, Calif.-based Northrop Grumman Aerospace Systems, which is leading the observatory’s design and development for NASA’s Goddard Space Flight Center in Greenbelt, Md. “All our budgets and schedules are based on this, and it’s a confirmation of the basic plan we proposed 10 years ago.”

JWST, a joint effort by NASA, the European Space Agency and the Canadian Space Agency, is a next-generation space observatory that will explore deep-space phenomena from distant galaxies to nearby planets and starts.

Lee Feinberg, Webb’s optical telescope element manager at Goddard, said completing the polishing of the primary mirror segment is “a hugely significant milestone for the Webb telescope project that demonstrates that our primary mirror segments can be completed on schedule while meeting the performance necessary for science.”

Cryogenic polishing, also known as cryo-null figuring, ensures that when the mirror reaches extremely cold operating temperatures in space, its shape will conform to the exact optical specification required to collect accurate infrared images of distant stars and galaxies, according to the news release.

The engineering development mirror, which will be used as a flight spare, was cryo-tested in the X-Ray and Cryogenic Facility at NASA’s Marshall Space Flight Center in Huntsville, Ala., while mirror polishing was performed at Richmond, Calif.-based Tinsley Laboratories.

Additional testing is planned as work continues to assemble the telescope segments.

“For validation purposes, we’re planning four sets of completely different cross checks and verification tests to authenticate the outcome of the mirror cryotests,” said Scott Texter, JWST optical telescope element manager at Northrop Grumman. “If any discrepancies surface, we can then investigate and reverify.”

Boulder, Colo.-based Ball Aerospace & Technologies Corp. will conduct separate verification tests using different computer-generated holographic null tools, while Goddard will use its own testing equipment and measurement methods in its clean room. Testing at NASA’s Johnson Space Center in Houston will use a reflective null tool manufactured by optical integration and test partner ITT, and Tinsley Labs will make measurements using its own independent method of calibrating its computer-generated holographic null tools, according to the release.