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Webb Telescope Engineers to Test COCOA

Photonics.com
Dec 2011
GREENBELT, Md., Dec. 22, 2011 — To verify the optical performance of the mirrors on the James Webb Space Telescope, NASA is using COCOA (center of curvature optical assembly) to check that the mirrors are perfectly shaped and will work in the frosty environment of space.

Curvature is important in a mirror, just as the convex side mirrors on your car are shaped to give you a wide field of vision behind and beside your car. COCOA tests on the Webb telescope’s concave mirror segments are critical because they will tell engineers if all of the mirrors are working together to make a telescope that has the correct shape.


The Center Of Curvature Optical Assembly (COCOA) will allow the program to verify the optical performance of the 21.3-ft (6.5 m) primary mirror at its 40-K (-233 ºC, or -387.4 ºF) operating temperature. The COCOA contains mechanical and optical instruments that allow the test team to identify, align and test the 18 segments from outside the vacuum chamber. Note: The background of this image has been digitally removed. (Image: ITT Exelis)

"We need to check that the mirrors are of the right prescription, just like eyeglasses, so the images from our telescope are not blurry," said Lee Feinberg, optical telescope manager at NASA's Goddard Space Flight Center.

The Webb telescope has 21 mirrors, with 18 of these being six-sided segments working together as one large 21.3-ft (6.5-meter) mirror. Each individual mirror has previously been tested to confirm that it has the correct shape, but testing them all together as an assembled telescope with COCOA ensures that the telescope as a whole works correctly.


NASA engineer Ernie Wright looks on as the first six flight-ready James Webb Space Telescope's primary mirror segments are prepped to begin final cryogenic testing at NASA's Marshall Space Flight Center. This represents the first six of 18 segments that will form NASA's James Webb Space Telescope's primary mirror for space observations. (Image: NASA's Marshall Space Flight Center/David Higginbotham)

The COCOA is part of NASA’s vacuum cryo equipment that will be used at NASA’s Johnson Space Center in Houston to test the performance of the mirrors at operating temperatures. That’s important because COCOA tells engineers whether the full 18-segment mirror is functioning correctly in operating temperatures of 40 K (-233 ºC, or -387.4 ºF) prior to final assembly of the observatory before launch.

COCOA was built by ITT Exelis of Rochester, N.Y., with subcontractor Micro Instruments, also in Rochester, N.Y. ITT Exelis and Micro Instruments engineers are assembling the large Center of Curvature test system.

For more information, visit: www.jwst.nasa.gov  


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