FEATURE
Zhang’s Glass Kitchen: Goddard Team to Begin NuSTAR MIrror Production this Fall
Astrophysicist Will Zhang and his team win
a position on NASA’s NuSTAR mission to
produce more than 3,000 curved mirrors
needed to focus high-energy X-ray photons.
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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
Telescopic Telescope Array (NuSTAR) to provide the telescope’s more
than 3,000 individual mirror segments.
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.”
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Testament to R&D Funding
As Zhang and his team gear up to begin the massive production job this
fall, he reflected 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 593ºC (1,100ºF). 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.”
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