The Mirror Makers- page 3 | Space | Air & Space Magazine

The Mirror Makers

The fight is on for the chance to build the world's most advanced space telescope.

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(Continued from page 2)

To reach space, the mirror will have to be sent atop an unmanned rocket, folded up like the leaves of a table or the petals of a flower. It must unfold in space without jamming. Then it has to hold its shape for 10 years in temperatures close to –457 degrees Fahrenheit, the point at which matter no longer has any thermal energy.

Engineers aren’t entirely sure what the extreme cold will do to the segments, and flaws invisible to the naked eye would blur the telescope’s images. The mirror will be supported by numerous tiny, electrically controlled mechanical arms, called actuators, which will, in a technique called adaptive optics, nudge each segment into place after deployment. Then, perhaps once a month, NGST will look out to a reference star. If the image is blurry, ground controllers will send commands up to the telescope to nudge the mirror segments back into place.

The project is so daunting that the potential contractors are relieved that NASA has reined in the program slightly. Earlier this year, agency officials signaled their willingness to settle for a telescope of six to seven meters instead of eight.

On the other hand, some astronomers are convinced that bigger is better, and they completely support Goldin’s vision of an eight-meter NGST mirror.

“In the early universe, there was a time when the first stars and star clusters lit up and illuminated the world, so to speak,” says astronomer Simon Lilly of the Herzberg Institute of Astrophysics in Victoria, British Columbia. He worries that a smaller mirror will produce degraded image quality and necessitate additional observation time. “We tried to design NGST so that observing that epoch is within its grasp,” he says. “For that, we need every little bit of sensitivity we can get. The bigger the better.”

Mather agrees there are significant advantages to a larger mirror. “Since the aim is to see the first light from the first objects that formed after the Big Bang, we don’t want to spend a year taking data with a four-meter telescope that would have taken less than a month with the eight-meter,” he says. “This is such a big difference that we don’t think the four-meter telescope will be able to see what we want to see…. We are pretty sure that a four-meter telescope would take 16 times as long to collect the light from a primordial galaxy as the original eight-meter concept would have done…. We can partially compensate for having a smaller telescope by getting more and better detectors.”

However, Dressler thinks a mirror with a minimum diameter of four meters would do just fine. He is nervous that NASA might be pushing the technology too far, which could leave astronomers with a flawed telescope or one that devours NASA’s astronomy budget as agency contractors try to reconcile the various elements required of the NGST mirror.

In Rochester, New York, another traditional American company with a homespun reputation is hoping to solve the puzzle. Kodak is the “dark horse” in the race, says Bernie Seery, NASA’s NGST program manager. “But the company is aggressively engaged in the selection process and is a formidable competitor.” Although Kodak is on the same team as Ball and will share a portion of the work if TRW is selected to build NGST, Kodak is still in competition with Ball to provide the mirror. Kodak engineers are working with a familiar concoction of melted silica used in the company’s cameras since the first Brownie. “Glass has been a traditional mirror material, so a lot is known about its manufacture,” says Jeffrey Wynn, general manager of space science in Kodak’s commercial and government systems division.

Kodak’s engineers hope their method of fashioning the mirror will result in one that is lighter and less expensive than a conventional glass mirror. “Ours is a little bit different,” says Wynn. “It’s a semi-rigid design constructed of a glass core section that looks like a honeycomb.” Glass is fused into a top and bottom plate while the mirror blank is still flat. “Then we slump it over a mandrel [a metal mold] and shape it.”

Such an approach eliminates much of the labor-intensive manufacturing processes usually required in building glass mirrors. “It’s much easier to work on mirrors in the flat than in steep curvatures,” says Wynn. “That’s what takes a lot of time—the steep curvatures in large parabolic mirrors have to be perfect, and the most laborious parts in generating those curves are the machining operations and the polishing.”

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