SOFIA Telescope Primary Mirror Receives Aluminum Coating

The main mirror for NASA's new airborne eye on the universe is now ready for installation after being transformed from a carefully shaped and polished piece of glass into a highly reflective optical component at Ames Research Center.

Project engineers completed the first mirror coating of the German-built telescope, a major project milestone, in a 10-ton, 16-foot tall stainless steel vacuum chamber at Ames. SOFIA optical engineers and scientists will annually re-coat the mirror, as is done for other large research telescope mirrors, and also routinely clean the mirror.

One of the challenges to a successful coating of the mirror was proper selection and testing of vacuum-compatible materials that support the large structure containing the mirror, known as the mirror cell. SOFIA is unique because the mirror cell is largely composed of carbon fiber reinforced plastic (CFRP), a material commonly found in tennis racquets and modern sailboat hulls. These types of composite materials provide the light weight and stiffness required for precision airborne optical components, but also easily absorb moisture from the air.

The SOFIA Primary Mirror Assembly contains 880 kg (1,950 pounds) of glass and more than 1100 kg (2,400 pounds) of CFRP. When the mirror and mirror cell were in the coating chamber and air was removed from the chamber, it took nearly a week for the CFRP to release its absorbed moisture, approximately half a liter (one pint). Afterthe moisture was removed, the pressure in the coating chamber was low enough for the mirror coating process to begin.

The coating chamber vaporizes aluminum by heating more than 60 tungsten filaments around its edges. These filaments have shapes similar to ones found in ordinary light bulbs, but are much larger. Each filament is laced with small twists of 99.999 percent pure aluminum wire.

The aluminum coating applied to the mirror glass is only .000150 millimeters thick (five one-millionths of an inch), approximately 1/300 of the thickness of a human hair, and weighs slightly more than two grams (1/14 of an ounce), equivalent to 1/7 of the metal in a soda can.

SOFIA is a joint NASA and German Space Agency, Deutsches Zentrum fur Luft- und Raumfahrt (DLR) program. The program is currently managed at NASA's Dryden Flight Research Center with the aircraft based at the Dryden Aircraft Operations Facility. NASA's Ames Research Center manages SOFIA science and mission operations in cooperation with USRA and the Deutsches SOFIA Institute (DSI).

Established in 1969 by the National Academy of Sciences, USRA is a private, nonprofit consortium of 102 universities offering advanced degrees in space- and aeronautics-related disciplines. USRA's mission is to conduct leading-edge research, develop innovative technologies, promote space-related education and policy, and operate premier science/technology facilities by involving universities, private industry and government.