The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint project of NASA and the German Aerospace Center (DLR) to construct and maintain an airborne observatory. NASA awarded the contract for the 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 (Deutsches SOFIA Institut) manages the German parts of the project which are primarily science and telescope related. SOFIA’s telescope saw first light on May 26, 2010. SOFIA is the successor to the Kuiper Airborne Observatory.

SOFIA is based on a Boeing 747SP wide-body aircraft that has been modified to include a large door in the aft fuselage that can be opened in flight to allow a 2.5 m (8.2 ft) diameter reflecting telescope access to the sky. This telescope is designed for infrared astronomy observations in the stratosphere at altitudes of about 12 kilometres (41,000 ft). SOFIA’s flight capability allows 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 will be available. The aircraft can also travel to almost any point on the Earth’s surface, allowing observation from the northern and southern hemispheres.

Once ready for use, observing flights were expected to be flown 3 or 4 nights a week. Originally scheduled to be operational for 20 years, in its tentative budget for the fiscal year 2015 NASA announced that unless Germany’s aerospace center would contribute significantly more than previously agreed upon, the observatory would be grounded by 2015. The SOFIA Observatory is based at NASA’s Neil A. Armstrong Flight Research Center at LA/Palmdale Regional Airport, California, while the SOFIA Science Center is based out of NASA Ames Research Center, in Mountain View, California.

SOFIA uses a 2.5 m (8.2 ft) reflector telescope, which has an oversized, 2.7 m (8.9 ft) diameter primary mirror, as is common with most large infrared telescopes. The optical system uses 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 is 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 is 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 is used for a camera guidance system.

The telescope looks 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 0.3–1.1 μm. The main instruments are the 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 include an optical photometer and infrared spectrometers with various spectral ranges. SOFIA’s telescope is by far the largest ever to be placed in an aircraft. For each mission one interchangeable science instrument will be attached to the telescope. Two groups of general purpose instruments are available. In addition an investigator can 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 detector arrays and to add the capability of measuring the polarization of dust emission from celestial sources.

The open cavity housing the telescope will be exposed to high-speed turbulent winds. In addition, the vibrations and motions of the aircraft introduce 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 includes a system of bearings in pressurized oil to isolate the instrument from vibration. Tracking is 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. The telescope cabin must be cooled prior to aircraft takeoff to ensure the telescope matches the external temperature to prevent thermally induced shape changes. Prior to landing the compartment is flooded with nitrogen gas to prevent condensation of moisture on the chilled optics and instruments.

DLR is responsible for the entire telescope assembly and design along with two of the nine scientific instruments used with the telescope, NASA is responsible for the aircraft. The manufacturing of the telescope was subcontracted to European industry. The telescope is German; the primary mirror was cast by Schott AG in Mainz, Germany with lightweight improvements, with grinding and polishing completed by the French company SAGEM-REOSC. The secondary silicon carbide based mirror mechanism was manufactured by Swiss CSEM. A reflective surface was applied to the mirror at a facility in Louisiana but the consortium now maintains a mirror coating facility in Moffett Field, allowing for fast recoating of the primary mirror, a process that is expected to be required 1-2 times per year.

The primary science objectives of SOFIA are to study the composition of planetary atmospheres and surfaces; to investigate the structure, evolution and composition of comets; to determine the physics and chemistry of the interstellar medium; and to explore the formation of stars and other stellar objects. While SOFIA aircraft operations are managed by NASA Dryden, NASA’s Ames Research Center in Mountain View, California, is home to the SOFIA Science Center which will manage mission planning for the program. On 29 June 2015, the dwarf planet Pluto passed between a distant star and the Earth producing a shadow on the Earth near New Zealand that allowed SOFIA to study the atmosphere of Pluto.

F/A-18 mission support aircraft shadows SOFIA during a functional check flight.

Wikipedia