The Stratospheric Observatory For Infrared Astronomy (SOFIA) was an 80/20 joint project of NASA and the German Aerospace Center (DLR) to construct and maintain an airborne observatory. NASA awarded the contract for development of the aircraft, operation of the observatory and management of the American part of the project to the Universities Space Research Association (USRA) in 1996. The DSI (German SOFIA Institute; ) managed the German parts of the project which were primarily science-and telescope-related. SOFIA's telescope saw first light on May 26, 2010. SOFIA was the successor to the Kuiper Airborne Observatory. During 10-hour, overnight flights, it observed celestial magnetic fields, star-forming regions, comets, nebulae, and the Galactic Center.
Science flights concluded after the landing of the 921st and final flight in the early morning of September 29, 2022. The Boeing 747SP that carried the telescope has been preserved and put on display at the Pima Air & Space Museum near Tucson, Arizona.
Facility
SOFIA was based on a Boeing 747SP, a factory shortened version of the wide-body aircraft that had been modified to include a large door in the aft fuselage that could be opened in flight to allow a diameter reflecting telescope access to the sky. This telescope was designed for infrared astronomy observations in the stratosphere at altitudes of about . SOFIA's flight capability allowed it to rise above almost all of the water vapor in the Earth's atmosphere, which blocks some infrared wavelengths from reaching the ground. At the aircraft's cruising altitude, 85% of the full infrared range was available. The aircraft could also travel above almost any point on the Earth's surface, allowing observation from the northern and southern hemispheres.
Observing flights were flown three or four nights a week. The SOFIA Observatory was based at NASA's Armstrong Flight Research Center at Palmdale Regional Airport, California, while the SOFIA Science Center was based in Ames Research Center, in Mountain View, California.
The telescope
thumb|The NASA logo reflected in SOFIAs 2.5-meter primary mirror.
SOFIA used a reflector telescope, which had an oversized, diameter primary mirror, as was common with most large infrared telescopes. The optical system used a Cassegrain reflector design with a parabolic primary mirror and a remotely configurable hyperbolic secondary. In order to fit the telescope into the fuselage, the primary was shaped to an f-number as low as 1.3, while the resulting optical layout has an f-number of 19.7. A flat, tertiary, dichroic mirror was used to deflect the infrared part of the beam to the Nasmyth focus where it can be analyzed. An optical mirror located behind the tertiary mirror was used for a camera guidance system.
The telescope looked out of a large door in the port side of the fuselage near the airplane's tail, and initially carried nine instruments for infrared astronomy at wavelengths from 1–655 micrometres (μm) and high-speed optical astronomy at wavelengths from 0.3 to 1.1 μm. The main instruments were FLITECAM, a near-infrared camera covering 1–5 μm; FORCAST, covering the mid-infrared range of 5–40 μm; and HAWC, which spans the far-infrared in the range 42–210 μm. The other four instruments included an optical photometer and infrared spectrometers with various spectral ranges. During its time in service, SOFIA's telescope was by far the largest placed in an aircraft. For each mission one interchangeable science instrument was attached to the telescope. Two groups of general-purpose instruments were available. In addition, an investigator could also design and build a special purpose instrument. On April 17, 2012, two upgrades to HAWC were selected by NASA to increase the field of view with new transition edge sensor bolometer detector arrays and to add the capability of measuring the polarization of dust emission from celestial sources.
The open cavity housing the telescope was exposed to high-speed turbulent winds. In addition, the vibrations and motions of the aircraft introduced observing difficulties. The telescope was designed to be very lightweight, with a honeycomb shape milled into the back of the mirror and polymer composite material used for the telescope assembly. The mount included a system of bearings in pressurized oil to isolate the instrument from vibration. Tracking was achieved through a system of gyroscopes, high-speed cameras, and magnetic torque motors to compensate for motion, including vibrations from airflow and the aircraft engines.