The Mirror Makers
The fight is on for the chance to build the world's most advanced space telescope.
- By Ben Iannotta
- Air & Space magazine, November 2001
(Page 6 of 6)
Engineers at Danbury say they know a thing or two about beryllium. In fact, they are openly resentful about the credit Ball engineers appear to be taking for SIRTF, which operates with a mirror fabricated in Danbury. “The bottom line is they’re flying the hardware that was made, in part, with these hands right here,” says Mark Stier, a blunt-speaking astronomer whose job, if Danbury wins the contract, would be to oversee the assembly of NGST’s mirror system. “There are only two places in the world where beryllium mirrors are made; this is one of them,” he says.
Stier is doubtful the metal is the first choice for the NGST mirror. The problem with beryllium is that its crystalline structure is not the same in all directions. “[The crystals] tend to expand more in one direction than another,” Stier says. “It’s not as homogenous as glass.” When beryllium hits the cold of space, it will shrink. “The mirrors would probably change their shape more when you cooled them than you would desire,” Stier says.
The Danbury team’s hard-won experience with SIRTF’s beryllium mirror has influenced the direction its NGST mirror is taking. “Did we have a hard time with [beryllium]? Yes. Are we concerned about beryllium? Yes. But that was not the determining factor in terms of our decision,” says Ira Schmidt, who managed the company’s Chandra program and is now in charge of the NGST work. “We know some of the idiosyncrasies of beryllium that maybe our competitors don’t know,” he adds.
But for both Danbury and Kodak, the more traditional material—which was used to fashion the Hubble’s mirror—will most likely be chosen. “We believe that glass works fairly well at cold temperatures,” says Kodak’s Jeffrey Wynn. “And programs like the space-based laser [prove] that glass stays in shape, depending on the application.”
And there is another huge advantage to glass. “Look at how many skyscrapers have large windows of glass,” Terence Facey says. “Glass is a well-understood material available in almost any size you want.” That is why the Danbury team believes that it can build the mirror from just nine segments, compared with the Ball team’s 36.
And Wynn thinks that a decision on the NGST mirror may not spell the end of the two options that aren’t selected as the final design. “Three different types of technology are still in development,” he says. “Danbury glass, Kodak glass, and Ball beryllium. The prime [contractor] and NASA may jointly make the selection, or they may carry two designs for a while as a backup. This is new technology no one has ever tried before.”
But in this field, today’s innovations quickly become tomorrow’s antiques. Even after the NGST is boosted to its lonely post at Lagrange point 2, both glass and beryllium may be passé. Seery and other NASA officials hope to start a program—the Large Telescope Systems Initiative—that would start looking at other lightweight materials for mirrors. But for now, either glass or beryllium, in a six- or seven-meter mirror, built either by old hands at mirror making or by a team new to the game, will offer scientists their farthest look yet back in time.