A few years ago, Goddard technologist Vince Bly participated in a series of brainstorming sessions looking at how NASA might use thin silicon wafers — like those used in Pentium IV computer chips — to build ultra-lightweight mirrors for space telescopes. Although it soon became clear to him that the idea probably wouldn’t work, it did get him thinking about silicon as a mirror material.
After 3 years of research and development, in part using Director’s Discretionary Funding, Bly’s idea paid off. Just a few weeks ago, he delivered the first two curved single-crystal silicon mirrors to Goddard Earth scientist Scott Janz who plans to use them in Geostationary Spectrograph (GeoSpec), an experimental instrument he’s developing under NASA’s Instrument Incubator Program (see related story).
Bly says he’s pleased so far with the mirror’s initial test results and optimistic that he’ll find other applications for his technology. He plans to conduct more validation tests at Goddard and the Marshall Space Flight Center and design additional single-crystal silicon mirrors of varying shapes and sizes. “Most likely, the best way to do this is to provide mirrors for other projects,” he said.
“My belief is that single-crystal silicon mirrors are a natural — especially for telescope spectrometers and similar instruments that operate in deep space and must endure super-cold temperatures,” he said, adding that those missions include everything from a Terrestrial Planetfinder to a large telescope that studies the very early universe.
The Appeal of Silicon
What initially attracted Bly to the material is its high-thermal conductivity and low expansion, which make it less likely to distort when exposed to extreme swings in temperature — especially compared with more traditional mirror-making materials like quartz or glass. In addition, silicon is extremely pure and crystalline, making it more uniform and stable. And last, silicon is commercially available. The semiconductor industry can now manufacture boules that are nearly 20 inches in diameter. “That’s certainly large enough for most optical instrument applications,” Bly said.
The question that remained was how to fabricate a lightweight mirror from the material.
Reversing the Process
In conventional lightweight mirror making, technicians remove portions of the mirror blank before they polish the optical surface. The technique creates a honeycomb that maintains the mirror’s strength while significantly reducing its weight. But with single-crystal silicon, which isn’t as stiff as other for lightweight mirrors, the approach wouldn’t work, Bly said. “If the light weighting were too aggressive, then the pattern of the supporting ribs would print through to the mirror surface,” Bly said. The resulting mirror would have a poor optical figure or would have to be significantly heavier.
But Bly had another idea. He figured that because single-crystal silicon is so pure and homogeneous, he could light weight the mirror after he polished the optical surface. This approach wouldn’t work with conventional materials because the removal of so much material would unbalance the material’s internal structure and distort the optical surface.
He has continued to improve the process and now his mirrors distort no more than about one-fiftieth wave after light weighting, which is good enough for even the most critical applications, he said. In fact, a recent flat mirror he created was so flat that if stretched to one mile in diameter, the highest bump on the mirror would be less that the width of a human hair.
The Office of the Chief Technologist is involved in a variety of projects, missions, and technologies.