FEATURE

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The Secret’s in the Sauce

Although some thought it couldn’t be done, a Goddard scientist has invented a technique to mold lightweight commercially available glass into high-quality mirrors capable of focusing X-rays and satisfying the demanding technical specifications of NASA’s next-generation Constellation-X.

To produce curved mirrors, Goddard contractors Marton Sharpe (front) and Jim Mazzarella place a sheet of very thin glass on top of the mandrel and place the assembly into an oven. The heat softens the glass so that it takes the shape of mandrel.

From a laboratory in Building 22, astrophysicist Will Zhang and two technicians are producing several mirrors a week in an effort to fine-tune a technique that Zhang hopes a private company will eventually use to mass produce the estimated 16,000 mirror pieces needed for Constellation-X, NASA’s next-generation X-ray observatory slated for launch in 2018. In contrast to the X-ray mirrors on the Chandra X-Ray Observatory, which are an inch thick, these Goddard-developed curved mirrors are only 400 microns thick. Contractor Jim Mazzarella lifts the glass from a mandrel he uses for cutting.

Like many X-ray telescopes, Constellation-X will use a Wolter Type I mirror design, distinguished by its nested mirrors that are curved so that highly energetic X-ray photons graze their surface, instead of passing through them — much like a stone skimming the surface of a pond. In particular, Constellation-X’s mirror will include four mirror assemblies, each containing 230 nested shells for a total of 16,000 mirror pieces. The nested configuration increases the mirror collection area, and therefore the telescope’s sensitivity.

Mirrors Measure Only 400 Microns

What stands out is the size of the actual mirrors. In contrast to the mirrors on the Chandra X-Ray Observatory, which are an inch thick, the mirrors specified for Constellation-X are only 400 microns thick. This “terrifically thin” mirror, however, cannot be shaped, polished and ground under traditional manufacturing practices.

Zhang’s technique offers a solution. In his laboratory, he uses flat sheets of smooth, lightweight glass measuring about 400 microns in thickness and places them on a mandrel or rounded mold that provides the exact optical prescription for Constellation-X’s mirrors. He then places the entire assembly inside an oven that heats the glass to about 1,400°F. As the glass heats, it softens and folds over the mandrel to produce a perfectly curved mirror. Eventually, he will be able apply the same process on bigger and thinner glass sheets.

The Secret The secret, Zhang said, lies in the preparation. “What we’ve learned is how to treat the mandrel and the glass sheet before we heat them in the oven,” he said. Though he cannot offer specifics for proprietary reasons, he said the approach preserves the mirror’s surface quality, which otherwise would be compromised in the process. Technicians then analyze the mirrors to determine their micro-roughness and to make sure that they are exact replicas of the mandrel itself. So far, the tests have revealed that the mirrors essentially meet the technical specifications required of Constellation-X. “We’ve shown that we can meet Constellation-X’s technical requirements. The skeptics are even saying that this is really possible,” Zhang said. “But the key is whether we can mass produce the mirrors.”

Marton Sharpe holds one of the curved mirrors he’s produced in the laboratory.