Zhang's Glass Kitchen
It pays to persevere. No one knows this better than Goddard astrophysicist Will Zhang.
After 10 years of fine-tuning a technique to efficiently manufacture super-thin, curved mirrors needed to focus X-ray photons, he and his team have won a position on the Caltech-led Nuclear Spectroscopic Telescope Array (NuSTAR) mission
to provide the telescope’s more than 3,000 individual mirror segments — a level of work estimated at $6.1 million.
More significant, however, is the longer-range potential for his mirror technology, he readily concedes.
"NuSTAR will be a precursor to Constellation-X," Zhang said, referring to the proposed flagship X-ray mission that NASA hopes to fly in 2018 to answer the most compelling unsolved problems in astrophysics and cosmology: what happens at the edge of a black hole? What is the mysterious dark energy that is pulling apart the universe? What powered the Big Bang? "This is really an opportunity for us to demonstrate that the technology is flight worthy and is ready for a bigger mission. Producing NuSTAR’s mirrors will let us show that we can mass produce the segments."
Testament to R&D Funding
As Zhang and his team gear up to begin the massive production job this fall, he reflects on how he eventually earned a position on the NuSTAR mission. Until just a few months ago, he wasn’t even on the team.
Initially, Caltech had chosen Columbia University to produce the mirror segments. In 2006, NASA cancelled the mission due to funding shortfalls. When it became obvious that the Agency needed the mission to bridge the gap between the 2009 launch of the Wide-field Infrared Survey Explorer and the 2013 launch of the James Webb Space Telescope, the Agency restarted NuSTAR in 2007. By then, Goddard had pulled ahead of Columbia in fine-tuning the production technique.
"There are two reasons why we pulled ahead," Zhang said. "Number one, we started out with the right idea; and two, we had more money and professionals than Columbia." Money and personnel were initially provided through Goddard’s Director’s Discretionary Fund (DDF) in 1998 and subsequently through the Internal Research and Development (IRAD) program in 2004-2007. Zhang’s team also received support from the Constellation-X project, he said.
"Without DDF and IRAD funding, our work would have taken much longer and we might have been beaten by the competition," Zhang said. "It sometimes takes a decade to bring an idea to the point where you have something to show. The R&D funding gave us time and money. It gave us encouragement and legitimacy. To get an idea like this going, you need both. Now, we’re talking about flying this by 2011."
A Good Idea
Just as important is the idea itself.
X-ray mirrors must be curved and nested inside an optical assembly so that the highly energetic X-ray photons graze their surface, instead of passing through them — much like a stone skimming the surface of a pond. To make these curved segments, Zhang will use flat sheets of smooth, lightweight glass measuring only 200 microns thick — the thickness of three sheets of paper.
His production team will place the commercially available glass on a mandrel or rounded mold that provides the exact optical prescription for NuSTAR’s mirrors. Technicians will then place the entire assembly inside an oven that heats the glass to about 593 degrees C (1,100 degrees Fahrenheit). As the glass heats, it softens and folds over the mandrel to produce a curved mirror that is an accurate copy of the mandrel’s surface.
In contrast, Columbia University slumped the glass into the mandrel, not over it, Zhang said. "Our approach is more controllable. Furthermore, we use a release layer that prevents the glass from sticking." This proprietary preparation technique preserves the mirror’s surface quality. "Our yield is almost 100 percent," he said.
Production Begins in the Fall
Production begins this October after Zhang’s team installs 10 large ovens in his laboratory. Once Goddard technicians finish the job in November 2009, they will ship the 3,120 pieces to the Danish Space Research Institute, which is responsible for coating the segments with 300 alternating layers of tungsten and silicon. From there, the coated mirror segments will be shipped back to Columbia University where they will be placed and aligned in a nested configuration inside two 10-meter long assemblies to form the telescope’s grazing-incidence optics.
"This will be a revolutionary mission," Zhang said. "It would have been a shame for Goddard to sit by and not be a part of it."
NASA's Goddard Space Flight Center