James Webb Telescope gets its wings

Mar 15, 2013 by J.d. Harrington
Technicians complete the primary mirror backplane support structure wing assemblies for NASA's James Webb Space Telescope at ATK's Space Components facility in Magna, Utah. ATK recently completed the fabrication of the primary mirror backplane support structure wing assemblies for prime contractor Northrop Grumman on the Webb telescope. Credit: Credit: Northrop Grumman/ATK

(Phys.org) —A massive backplane that will hold the primary mirror of NASA's James Webb Space Telescope nearly motionless while it peers into space is another step closer to completion with the recent assembly of the support structure's wings.

The wings enable the mirror, made of 18 pieces of , to fold up and fit inside a 16.4-foot (5-meter) fairing on a rocket, and then unfold to 21 feet in diameter after the telescope is delivered to space. All that is left to build is the support fixture that will house an integrated , and technicians will connect the wings and the backplane's center section to the rest of the observatory. The center section was completed in April 2012.

"This is another milestone that helps move Webb closer to its date in 2018," said Geoff Yoder, NASA's program director, NASA Headquarters, Washington.

Designed, built and set to be tested by ATK at its facilities in Magna, Utah, the wing assemblies are extremely complex, with 900 separate parts made of lightweight graphite using advanced fabrication techniques. ATK assembled the wing assemblies like a puzzle with absolute precision. ATK and teammate of Redondo Beach, Calif., completed the fabrication.

This is a September 2009 artist concept of the James Webb Space Telescope. Credit: Credit: NASA

"We will measure the accuracy down to nanometers—it will be an incredible engineering and manufacturing challenge," said Bob Hellekson, ATK's Webb Telescope program manager. "With all the new technologies that have been developed during this program, the Webb telescope has helped advance a whole new generation of highly skilled ATK engineers, scientists and craftsmen while helping the team create a revolutionary telescope."

When fully assembled, the primary mirror backplane support structure will measure about 24 feet by 21 feet and weigh more than 2,000 pounds. The backplane must be very stable, both structurally and thermally, so it does not introduce changes in the primary mirror shape, and holds the instruments in a precise position with respect to the telescope. While the telescope is operating at a range of extremely cold temperatures, from minus 406 to minus 360 degrees Fahrenheit, the backplane must not vary more than 38 nanometers (about one one-thousandth the diameter of a human hair). The thermal stability requirements for the backplane are unprecedented.

"Our ATK teammates demonstrated the thermal stability on test articles before building the wing assemblies with the same design, analysis, and manufacturing techniques. One of the test articles ATK built and tested is actually larger than a wing," said Charlie Atkinson, deputy Webb Optical Telescope Element manager for Northrop Grumman in Redondo Beach, Calif. "The mirrors are attached to the wings, as well as the rest of the backplane support structure, so the alignment is critical. If the wings distort, then the mirror distorts, and the images formed by the would be distorted."

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User comments : 10

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baudrunner
2.7 / 5 (7) Mar 15, 2013
I hope I live long enough to see it happen. Really! It's been in development since the initial planning stage in 1996. No doubt its launch date will continue to be moved forward, currently set for 2018, a 'mere' 22 years after inception. Sheesh!
GuruShabu
1.6 / 5 (8) Mar 15, 2013
So do I, Baudrunner!
I've been waiting for this telescope for so long now.
My concern, besides all technicalities, is the Solar wind and the thermal planes.
How the telescope will be aimed towards a point and stay pointed.
Does it have to be steered all the time?
If so, for how long is the fuel to last?
fmfbrestel
5 / 5 (3) Mar 15, 2013
Steering is accomplished with reaction wheels powered by the solar panels on the other side of the sunshade. Solar panels do degrade with time, but the telescope will be obsolete long before the power degrades too far for accurate pointing.
fmfbrestel
5 / 5 (3) Mar 15, 2013
It does have some station keeping thrusters used to maintain it's orbit around L2. Only 2–4 m/s per year estimated thruster use, from the total budget of 150 m/s. Even at the high end, that's 30 years of station keeping.
GuruShabu
1.3 / 5 (7) Mar 16, 2013
Steering is accomplished with reaction wheels powered by the solar panels on the other side of the sunshade. Solar panels do degrade with time, but the telescope will be obsolete long before the power degrades too far for accurate pointing.

Steering to prevent drifting is necessary. Reaction wheels are gyroscopes that can keep the structure aimed towards a target (as you say "accurate pointing" but they cannot prevent translation...
rah
1 / 5 (5) Mar 16, 2013
What if, upon launch, a piece of tape is covering a sensor which causes the entire thing to splash into Biscayne Bay?
Fisty_McBeefpunch
1 / 5 (6) Mar 16, 2013
I was wondering how the mirrors are protected from micro meteors and other particles?
fmfbrestel
not rated yet Mar 19, 2013

Steering to prevent drifting is necessary. Reaction wheels are gyroscopes that can keep the structure aimed towards a target (as you say "accurate pointing" but they cannot prevent translation...

Reaction wheels and gyros are separate systems. Web actually uses Hemispherical Resonator Gyros (http://www.jwst.n...l#gyros) which do not wear out nearly as fast as conventional gyroscopes, but are used solely for sensing the orientation of the telescope, not for maintaining orientation. Corrections in orientation are done with reaction wheels.

Translation is corrected with the on board thrusters, but is only necessary for orbital corrections. Translation errors in relation to Web's targets is entirely inconsequential. 5m traslation error on a target light years away and (in some cases) light years across, do not matter.
fmfbrestel
not rated yet Mar 19, 2013
I was wondering how the mirrors are protected from micro meteors and other particles?


I don't think there are protection systems for these threats. However there are many fewer micro meteors at L2 than in LEO or GEO. Any strikes would be detected though and software correction in image processing may be able to minimize any impact on science observations.
GSwift7
1 / 5 (1) Mar 19, 2013
I was wondering how the mirrors are protected from micro meteors and other particles?

I don't think there are protection systems for these threats. However there are many fewer micro meteors at L2 than in LEO or GEO. Any strikes would be detected though and software correction in image processing may be able to minimize any impact on science observations.


There is a size threshold for little dings at which point it won't show up in images. Each individual CCD on the photographic sensor uses a given surface area of the mirrors. As long as the damage only takes out a portion of that mirror area, that individual CCD pixel should still function, just with reduced light gathering ability. The important thing to protect are the CCD sensors themselves. The mirrors are somewhat robust compared to the CCD's, which should be in a sealed enclosure. Even if you lose a bit of mirror or a CCD goes out, it's not the end of the world. It would be annoying, like losing a pixel on a TV.