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Stratospheric Observatory for Infrared Astronomy approaches first light

SOFIA, a new airborne telescope, is about to embark on 20 years of discoveries.
6 October 2009, SPIE Newsroom. DOI: 10.1117/2.1200909.1802

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is in the final stages of development. The project is a joint endeavor of NASA and the German space agency, ‘Deutsches Zentrum für Luft und Raumfahrt,’ to develop and operate a 2.5m airborne IR telescope aboard a Boeing 747SP (see Figure 1). Flying in the stratosphere, SOFIA will enable observations throughout the IR and submillimeter spectral regime (0.3μm to 1.6mm wavelength range), with an average transmission of >80%. First science flights will begin in 2010, and the observatory is expected to operate for more than 20 years.

Figure 1. NASA's Stratospheric Observatory for Infrared Astronomy (top) with F/A-18 safety chase on 10 May 2007 during the first series of test flights for performance verification of the modified Boeing 747SP aircraft and (bottom) during night-time flightline operational tests of the telescope in March 2008. (Images courtesy of NASA's Dryden Flight Research Center photo collection.)

SOFIA will be a key element in the astronomical community's research portfolio for chemical and dynamical studies of the interstellar medium and regions of star formation, observations of deeply embedded sources, and time-critical investigations of transient events. It will join the Spitzer Space Telescope,1,2 Herschel Space Observatory,3 and James Webb Space Telescope (JWST)4 as one of the premier facilities for astronomical observations at thermal-IR and submillimeter wavelengths. The facility can be upgraded continually and used as a test bed for state-of-the-art and high-risk technologies in conditions close to those encountered in space flight. In addition, it will excel as a training opportunity for a new generation of instrumentalists and experimental astronomers.

The observatory's first-generation instrument suite includes high-speed photometers, broadband imagers, moderate-resolution spectrographs capable of resolving broad spectral features caused by dust and large molecules, and high-resolution spectrometers suitable for kinematic studies of molecular and atomic gas lines at kilometer-per-second resolution. This complement of equipment should enable unique contributions to a broad array of science topics.

SOFIA will fly as high as 45,000ft (13.7km), above 99.8% of the obscuring atmospheric water vapor. At this altitude, precipitable atmospheric water typically has a column depth of less than 10μm, which is 100× lower than at good ground-based sites (see Figure 2).

Figure 2. Typical atmospheric transmission (blue) at an altitude of 45,000 feet (left) compared to that on a good night on Mauna Kea (HI, 13,800ft above mean sea level: right). From 1 to 1000μm, the average transmission is ≥80% except in the center of absorption lines of mostly H2O, CO2, and O2. Background image: Spitzer Space Telescope/IR Array Camera false-color image of the Sombrero galaxy. (Courtesy of NASA/Jet Propulsion Laboratory/California Institute of Technology.)

The observatory's deployment flexibility will enable measurements of transient events that are only visible from specific locations (such as stellar occultations) and of objects at extreme declinations (such as the Magellanic Clouds, our nearest neighbor galaxies). The rapid turnaround time between flights will facilitate timely observations of transient events, such as variable stars, comets, occultations, and (super)nova explosions.

The finished Nasmyth telescope (a modified form of a Cassegrain telescope on an altitude-azimuth mount), supplied by the German space agency, has been installed and subjected to a series of tests. All major structural modifications to the aircraft were completed in early 2006, and first test flights began in April 2007. The primary mirror was coated during the summer of 2008, and the aircraft is now based at NASA's Dryden Aircraft Operating Facility in Palmdale (CA), where closed-door flight testing will soon take place. The first open-door flights will begin in late 2009. SOFIA will see ‘first light’ in January 2010 and is planned to make more than 120 eight- to 10-hour scientific flights per year for at least 20 years. With our ability to deploy new and updated instruments, the observatory will play an important role in studies of a variety of astrophysical problems well into the 21st century.

Eric E. Becklin
Universities Space Research Association
Mountain View, CA

Eric Becklin received his BS and PhD degrees in physics from the University of Minnesota (1963) and the California Institute of Technology (1968), respectively. He is a professor emeritus of physics and astronomy at the University of California at Los Angeles. He served as the first director of the NASA Infrared Telescope Facility on Mauna Kea (HI) and as principal investigator of the Kuiper Airborne Observatory. He is widely known for his discovery, with Gerry Neugebauer, of an exceptionally bright IR source in the Orion Nebula, known today as the Becklin-Neugebauer object.

Robert D. Gehrz
Department of Physics and Astronomy
University of Minnesota
Minneapolis, MN

Robert Gehrz received both a BA (1967) and a PhD (1971) in physics from the University of Minnesota. From 1972 to 1985, he was a faculty member of the University of Wyoming where, in collaboration with John A. Hackwell, he built the 2.34m Wyoming Infrared Observatory. Since 1985, he has been a professor of physics and astronomy and director of the observatories at the University of Minnesota, and department chair since 2005.