The Mirror Makers

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

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Kodak’s AMSD experience produced a mirror in the ultralight range of less than 33 pounds per square meter, far lighter than Hubble’s massive 330-pound-per-square-meter mirror. Because of the size needed for the NGST mirror, Wynn hopes Kodak’s manufacturing techniques will prove that glass is the right choice.

But besides the choice of materials for the mirror, nearly every aspect of the telescope will prove difficult to create. “You’ve got to push people to do the very best they can, and at some point you’ve got to say ‘Okay…now I’ve got to really build it,’ ” Danbury’s Facey says. He notes that the same thing happened with the Hubble, which was originally slated to have a three-meter mirror.

Inside Danbury’s mirror manufacturing facility, Facey points to a giant 4.3-meter mirror that is resting horizontally on a metal support. He says it weighs 7,000 pounds and must be moved on a cushion of air.

NGST will be nothing like that. Under the current design, Danbury would fabricate nine segments that would be less than 20 millimeters thick—about two-thirds of an inch. They will be, in a word, flimsy. “You breathe on this thing and it’s going to change,” Facey says.

Danbury’s design calls for a central mirror that would be shaped like an octagon. Keystone-shaped segments would extend out from each flat side of this central mirror to create one large surface. The keystones would be hinged so that one could be folded forward, the next aft, and so on, around the central mirror. That is how the engineers plan to squeeze it inside an unmanned rocket’s payload shroud. The tolerances will be tight—Danbury engineers have spent a considerable amount of time studying the lessons of the Hubble program, in which the error resulted from engineers relying on a single measuring tool, which turned out to be flawed.

And the complexities of the mirror itself are greater. In contrast to Hubble’s single-piece mirror, Ball’s initial blueprint calls for 36 segments. Each must stay perfectly matched to the surrounding segments or the mirror’s delicate focus will be lost. That job will be more complex with more segments, Facey says. “It’s not a non-trivial issue to poke those optics with a couple hundred actuators and still get something that looks good,” he observes.

It does not take long to realize that each company has a radically different opinion about how many segments will be needed to make up the NGST mirror. Danbury figures at most nine, including the central mirror; Ball figures up to 36; Kodak’s AMSD mirror, while not strictly a prototype of its NGST design, uses 19.

Ball engineers are eyeing beryllium, the same metal that Danbury had so much trouble using for SIRTF’s primary mirror. It is highly reflective and would need no special coating. Glass mirrors, on the other hand, are coated with gold to make them reflective.

Beryllium, unlike glass, is not available in large disks, so a large beryllium telescope would have to be assembled from lots of smaller pieces. Yet the Ball team remains tempted by beryllium because it is much stiffer than glass, says Doug Neam, a former college wrestler who is now Ball’s NGST program manager. “One of the fundamental building blocks of this program is how you will get a mirror of extremely lightweight design,” he says. “That’s what got us into looking at beryllium mirrors.” Neam gestures toward a model of Ball’s proposed NGST mirror. “If you look at the back of this mirror here you will see that the actual face sheet is just a couple of millimeters thick. And then the mirror itself is a couple of inches think. Call it 50 millimeters-ish,” he says. The model is not to scale, but his point is that a beryllium mirror would be much lighter than an equivalent glass mirror.

Despite all the potential advantages, Ball astronomer Douglas Ebbets cautions that Ball has not yet ruled out glass. “That final decision won’t be made for another year, pending the outcome of tests of mirror technology developments that are currently under way,” he says.

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