The Mirror Makers- page 2 | 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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The three companies have been experimenting with different mirror materials through a $20 million project called the Advanced Mirror System Demonstrator (AMSD), which is funded jointly by NASA, the Air Force, and the National Reconnaissance Office, which operates spy satellites. The three companies each received about $3 million to build competing versions of lightweight mirrors and test them in vacuum chambers.

Each contractor will have to show that mechanical actuators on the back of the mirror can adjust the instrument’s shape, or “figure.” Phasing experiments, in which the contractors will prove that different segments can work together without losing focus, will come later.

For the contractors, the creation of the Hubble’s primary mirror provides a benchmark and, of course, a cautionary tale. It was here at Danbury, back in 1980, that Hubble’s primary mirror was ground and polished very precisely to the wrong specifications. The mirror’s curve was off by just one-fiftieth of the width of a human hair, but that was enough to spread Hubble’s light across multiple focal points instead of one. Here is the plot twist: The engineers and physicists at Ball, Danbury’s rival in Colorado, are the same happy few who stepped in to build the optics that corrected the images bouncing off Hubble’s flawed mirror.

Danbury officials deny that the real reason they are chasing the NGST contract is to redeem the company’s reputation. Facey insists the Hubble error was an “anomaly” in a long list of groundbreaking company triumphs, which includes the new Chandra X-ray observatory and the soon-to-be-launched Space Infrared Telescope Facility. Chandra is giving astronomers unprecedented information about black holes, and SIRTF will obtain images of not only the early universe but also planets in our solar system and the cosmic dust and gas surrounding nearby stars.

Ball, Danbury, and Kodak have teamed up with California-based aerospace giants that know how to integrate sensitive optical equipment and powerful satellite frames. Danbury is working with Lockheed Martin of Sunnyvale, and Ball and Kodak are joined up with TRW of Redondo Beach. But the prime NGST contractor must select the best mirror, regardless of any teaming arrangements—otherwise, NASA won’t approve the design.

Out in Boulder, in the same low-slung buildings at the foot of the Rocky Mountains where Hubble’s corrective optics were built, members of the Ball team make it clear that the competition is a fierce one.

Unlike Danbury, Ball—the same company that made your grandmother’s canning jars—is not known as a maker of large optical systems like Hubble’s primary mirror, let alone one that would be 10 times bigger. Its niche has been the design and manufacture of small scientific instruments. It made most of the instruments and cameras that sit behind Hubble’s main mirror, converting raw light from the mirror into pictures of the heavens. Because of schedule delays, Ball got the nod to build the SIRTF telescope and instruments through subcontractors after NASA halted Danbury’s work on that telescope’s 4.9-meter mirror.

NASA officials want the NGST mirror to be six to seven meters wide. A mirror that size could not possibly be ground from a single piece of glass, the way Hubble’s 2.4-meter mirror was, and must be made from segments. In order to stay within the telescope’s 6,600-pound payload limit, these need to be seven to 10 times lighter per square meter of surface area than Hubble’s mirror. NASA wants the new mirror to weigh no more than 44 pounds per square meter.

The size of the mirror—more than double that of Hubble’s, larger than any conceived of for a spacecraft mirror—started as a bold challenge from NASA Administrator Dan Goldin. In January 1996, Goldin stood up in front of the American Astronomical Society in San Antonio, Texas, and flabbergasted the audience by calling for the Next Generation Space Telescope to be built with a mirror eight meters across.

Carnegie’s Alan Dressler remembers sitting in the audience. He had just finished leading a meeting of astronomers who hoped to whet NASA’s appetite for a successor to Hubble. The Dressler Committee report, “The Hubble Space Telescope & Beyond,” called for a telescope with a mirror at least four meters across, or two-thirds wider than Hubble’s—half the size of Goldin’s proposed mirror. Dressler figured that was about the right size to magnify infrared waves and capture images of the very early universe. A bigger telescope would improve the resolution of the images somewhat, but the main goal was to get a mirror to the Lagrange point, far from Earth’s heat, where the infrared waves would stand out against the cold background of space, he says.

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