SOFIA Telescope Primary Mirror Receives Aluminum Coating

Photos below

June 20, 2008

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.

After years of development and preparation, it took just 20 seconds to apply the shiny, aluminum coating to the glass mirror for the Stratospheric Observatory for Infrared Astronomy (SOFIA). The telescope is scheduled to begin observations in mid-2009.

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). After the 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 comprised of a heavily modified Boeing 747SP aircraft that will carry a 2.5-meter (98-inch) diameter telescope into the stratosphere to conduct astronomical research. SOFIA is a joint venture of NASA and the German Aerospace Agency, Deutsches Zentrum fur Luft- und Raumfahrt (DLR). The SOFIA program is managed by NASA's Dryden Flight Research Center, Edwards, California, and the aircraft is based at the NASA Dryden Aircraft Operations Facility, Palmdale, California.

USRA technicians apply an acid solution to the uncoated SOFIA primary mirror, as the second of a 3-step cleaning process preparing the mirror to receive its final reflective aluminum coating that will transform it into a world-class scientific instrument. (Ken Bower)	BEFORE: View from inside the mirror coating chamber of the SOFIA primary mirror mounted face-down in the chamber roof, ready to receive its aluminum coat. The central circular plate holds test slides by which the quality of the coating can be checked. (Ken Bower)
AFTER: Two USRA technicians on the floor of the mirror coating chamber, photographing themselves reflected in the perfectly coated mirror suspended above them. (Patrick Waddell)	SOFIA's primary mirror being "light-weighted" during manufacture in France, carving out as much mass as possible while leaving the mirror structurally rigid.
Ed Schilling of NASA-Ames' AV department filming telescope motions in the uncovered cavity, near sunset August 25, 2004.	Sky light seen through the 2.7-meter SOFIA primary mirror. Once the mirror receives its opaque aluminum coating this pretty view will be impossible.