Indomitable Astrophysicist Seeks to Revolutionize X-ray Astronomy
Pursues Two Technologies for Advanced Mirror Fabrication
Will Zhang is at it again.
The Goddard astrophysicist last year produced and delivered 9,000 super-thin, curved glass mirrors for NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) mission using a novel manufacturing technique he developed with Goddard R&D funding.
The indomitable Zhang now has embraced a new challenge.
He has launched two parallel technology-development programs both aimed at developing the world’s first lightweight X-ray mirror that offers high-angular resolution and a photon-collection area 10 times larger than the current state-of-the-art — all at a reduced cost.
“The trick for scientists is finding the sweet spot,” Zhang said. “NASA’s Chandra had a thick, heavy mirror with a very small collecting area, but it had good angular resolution. The European Space Agency’s XMM-Newton had a larger collecting area for the same mass, but it traded off the angular resolution. And the Japanese, in their Suzaku mission, went for a small, light telescope with a large collecting area relative to its weight. But it, too, sacrificed resolution. You need all three to get a perfect mission. Our technology will realize good angular resolution and large collecting area on one lightweight telescope.”
In 2011, Will Zhang delivered 9,000 curved mirror segments for the Nuclear Spectroscopic Telescope Array mission, which NASA is launching later this year. He has since accepted a new challenge — developing new fabrication techniques to create more capable, less expensive X-ray mirrors. (Photo Credit: Chris Gunn)
Goddard’s Internal Research and Development (IRAD) and NASA’s Physics of the Cosmos programs are funding the first effort, which leverages technology Zhang employed to manufacture NuSTAR’s mirrors. The other, supported by NASA’s Astronomy and Physics Research and Analysis program, is more ambitious. Instead of fabricating mirror segments with glass, which is traditional, Zhang would create them from large blocks of mono-crystalline silicon — the same material used for making computer chips.
The goal is to advance their technology-readiness levels to the point where Zhang could offer either of the two for small missions in the near term and at least one large flagship mission in the next decade.
With NuSTAR, Zhang proved it was possible to develop modest-resolution, low-cost mirrors — not an insignificant undertaking given the difficulty of constructing these highly specialized mirrors measuring only 200 microns thick — about 100 times thinner than Chandra’s mirrors.
To collect X-rays, the mirrors must be curved and nested inside an optical assembly so that highly energetic photons graze their surface, instead of passing through them — much like a stone skimming the surface of a pond. To make these segments, Zhang used flat sheets of smooth lightweight glass and placed them on a mandrel or rounded mold that provided NuSTAR’s exact optical prescription.
He and his team then placed the entire assembly inside an oven that heated the glass to about 1,100 degrees Fahrenheit. As the glass heated, it softened and folded over the mandrel to produce a curved mirror. Colleagues at the Danish Technical University in Copenhagen coated the mirrors with layers of silicon and tungsten to maximize their X-ray reflectance.
“Precision slumping of thin glass sheets has shown to be an excellent way of making lightweight, higher-resolution X-ray mirrors, as evidenced by the delivery of the NuSTAR mirrors,” Zhang said. But the resolution can be even better.
The secret lies in aligning and assembling the many mirror segments into a higher-precision mirror module, he said. Zhang is leading a team to develop a new system, which he believes will improve the angular resolution of the slumped-glass optics by a factor of five. With IRAD support, he has constructed the module and plans to complete testing by the end of the year. The goal is achieving a technology-readiness level of five — the level typically required for a successful Explorer-class mission proposal — by December 2012.
At the same time, Zhang is working with Goddard technologist Vince Bly to construct super-thin X-ray mirrors from large blocks of mono-crystalline, a commercially available material never before polished and figured for making X-ray mirrors.
The key lies with the material itself. Traditional materials used for making mirrors — glass, ceramics, and metals — suffer from high internal stress, especially when cut or exposed to changing temperatures. These stresses become increasingly unpredictable as the mirror becomes thinner. “When I cut glass, I may not be able to predict how it will behave,” Zhang said, comparing it to plywood, which can distort after being cut into pieces.
But silicon, on the other hand, “is a perfect material,” Zhang said. “Every atom is in the right place. If plywood were made of silicon, it would be flat; even after cutting it, it wouldn’t distort.”
Because of the material’s stability, technologists would be able to polish the silicon first and then lightweight it by machining away the excess material. Currently, mirror makers do the exact opposite. They grind first and then polish mirror surfaces — a time-consuming and expensive process, Zhang said.
Under his three-year APRA, Zhang and Bly plan to first make very thin flat mirrors to demonstrate the method. In the second year, he will fabricate curved mirror segments whose resolutions compare to or exceed those of Chandra. By the third year, he will attempt to fabricate integrated mirror segments from the same block of silicon, which would eliminate the complexity of aligning and integrating the segments into a module.
If successful, the application could revolutionize X-ray astronomy and make it even faster and less expensive to fabricate high-quality mirrors, Zhang said.
“It takes a lot of money to launch spacecraft into orbit. Driving down costs permeates everything we do,” Zhang said. “The key is developing a lightweight, high-resolution mirror that that costs much less to manufacture. It is essential to the future of X-ray astronomy in particular and to the well-being of NASA’s science enterprise in general.”
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