New Process Developed for Telescope Mirror Manufacture

Dr. James P. Smith

Advanced Optical Systems Inc. has teamed with Oak Ridge National Laboratory in Tennessee to develop a promising alternative means to manufacture lightweight mirrors for telescopes used in space and military applications. Traditional manufacturing materials include beryllium and silicon carbide, but their fabrication involves environmental risks and special tooling. The key to the new process is its use of existing technologies such as casting, electroless nickel plating and precision machining to produce the lightweight aluminum-silicon alloy mirrors.

According to the researchers, the process is less expensive than traditional fabrication methods, and the mirrors have stiffness-to-weight ratios comparable to that of traditional products. They also exhibit low image distortion and low light scattering.

Advanced Optical Systems Inc. and Oak Ridge National Laboratory have developed a new process for fabricating lightweight telescope mirrors for space and military applications.

The casting for the mirror is anasil, an aluminum alloy that contains about 22 percent silicon. The electroless nickel surface coating matches Vanasil's coefficient of thermal expansion to within 5 percent. Advanced Optical produced a 13.5-kg/m², 6-in.-diameter mirror that is flat to 1 λ (0.2 λ rms) with no heat treatment. Many known techniques, such as heat treating, thermal cycling and finite element analysis, can improve the mirror's merit characteristics. Oak Ridge provided single-point diamond turning and achieved surface microroughness of 3 to 4 nm rms.

Lead scientist at Advanced Optical, Thomas Cantey, said, "Our goal is an areal density less than 6 kg/m² and further improvements in material properties. The new technology has only begun to scratch the surface and prove its merit among the well-budgeted beryllium and silicon carbide mirror technologies." The company is prepared to build a 0.6-m primary mirror and the complete telescope using the new technology.

"Although selection for funding the Phase II [Small Business Technology Transfer grant] follow-on is still pending," Cantey said, "we are confident that this technology has many commercial applications. We have been contacted by laser and spectroscopy companies who are interested."