17 Jun 2002
A technique for machining a mirror to reduce its weight is helping the world's largest airborne observatory become a reality.
The 'lightweighting' process, carried out by engineers at the French company, REOSC, has reduced the weight of a 2.7 metre mirror by nearly 80%. This process makes it is light enough to carry out observations from a modified Boeing 747SP aircraft flying at nearly 41,000 feet while remaining strong enough to withstand the atmospheric conditions at that height. The mirror is part of the telescope for NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) project, which should become operational in 2002.
The SOFIA observatory will be based at NASA's Ames Research Center. The German Aerospace Center (DLR) is responsible for the design and construction of the SOFIA telescope and will receive 20% of SOFIA's observing time in return.
This is the largest telescope mirror ever to be lightweighted using a mechanical cutting process. This process took nearly 18 months and reduced the primary mirror's weight from 4,500 to 880 kilograms. A high-precision milling machine produced honeycomb-like pockets and holes in the back of the mirror. These holes do not penetrate the front surface of the mirror.
The telescope, which will weigh about 20,000 kilograms in total, will operate primarily in the infrared region of the spectrum, but will also study visible light. The height of the telescope will allow astronomers to avoid more than 99% of the infrared-absorbing water vapour that limits ground-based observations. The telescope should help to study black holes, galactic evolution and the chemical composition of interstellar gas clouds.