SOFIA is a collaboration between NASA and the German Space Agency DLR to provide a new airborne observatory with much greater sensitivity than its predecessor, the Kuiper Airborne Observatory (KAO) described in Chapter 7.

The SOFIA aircraft is a Boeing 747SP aircraft, named the Clipper Lindbergh, which has been modified to accommodate a 2.5m reflecting telescope (Figure 8.4). Work began in 1996 and SOFIA made its first test flight in 2007. When it begins operations in 2009 SOFIA will be the largest airborne observatory in the world, making observations that are impossible for even the largest and highest of ground-based telescopes. Like the KAO, SOFIA's science operations will be managed by the NASA Ames Research Center, but the aircraft itself will be based at NASA's Dryden Flight Research Center, Edwards, California. The aircraft will cruise at an altitude of between 12,400 m and 13,700 m (41,000-45,000 feet) for periods of 8 hours at a time and will make approximately 160 flights per year for a total operational lifetime of 20 years.

During operations the SOFIA telescope will be at the ambient temperature of about —35°C (240 K) and will be equipped with a number of cryogenically cooled instruments to investigate IR radiation over a very wide range from 0.3 ^m to 655 ^m. The telescope will view out of one side of the aircraft over a range of elevation angles from +20° to +60°. The telescope will be isolated against aircraft

Figure 8.4. The SOFIA airborne observatory aircraft. Courtesy of NASA/DLR.

vibrations by air bladders (or air springs) and will be further stabilized by gyroscopes, which will counter sudden gusts from outside the 4 m wide opening in the aircraft into the telescope cavity. In flight, air flows past this door at 800 km/h and so observations at visible wavelengths will be somewhat blurred by the accompanying turbulence. However, observations are affected less and less as one goes to longer wavelengths and seeing is expected to be diffraction-limited at wavelengths greater than 15 ^m.

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