Making unique IR observations with an airborne 2.5m telescope

The Stratospheric Observatory for Infrared Astronomy operates from a modified Boeing 747 and enables unique spectroscopy and imaging not possible with ground-based IR observatories.
23 August 2016
Eric E. Becklin, Maureen L. Savage, Erick T. Young and Dana E. Backman

Large parts of the IR spectrum are inaccessible in observations made from ground-based telescopes because of absorption by water vapor in the atmosphere.1 For this reason, the Stratospheric Observatory for IR Astronomy (SOFIA)—a joint project between NASA and the German Aerospace Center (DLR)—was designed and has been operational since 2010.2 The observatory consists of a gyro-stabilized telescope (with an effective diameter of 2.5m), mounted within a Boeing 747SP (special performance) aircraft (see Figure 1) that flies at altitudes as high as 45,000 feet (13.7km). At such altitudes, the atmospheric water vapor overburden typically has a column depth of less than 10μm, which is 20–100 times lower than at good terrestrial sites. Indeed, the average atmospheric transmission is 80% or more across SOFIA's wide wavelength range (0.3μm to 1.6mm), as shown in Figure 2. Although some strong water absorption lines remain in the spectra, high-quality spectroscopy and broadband photometry can be achieved between the telluric (i.e., atmospheric) lines. SOFIA has thus become a key facility for several astronomy investigations, e.g., for studying regions of star formation, observing objects obscured by interstellar dust, and making time-critical measurements of transient events.

Figure 1. Top: Photograph of the Boeing 747 special performance aircraft that houses the Stratospheric Observatory for Infrared Astronomy (SOFIA). This image was obtained from a chase airplane, during a door-open test flight in 2010. Bottom: SOFIA's 2.5m-diameter primary mirror. The aft ramp, which minimizes circulating wind (caused by airflow over the open cavity), is shown on the right. (Images courtesy of NASA.)

Figure 2. Calculated atmospheric transmission for SOFIA (black) at 13km compared with that of a hypothetical telescope (at an altitude of 5.6km) on the Cerro Chajnantor peak in Chile (red). In these calculations,110 and 700μm precipitable water vapor levels are assumed, respectively. The average SOFIA transmission between 20μm and 1.3mm is 80%. (Reproduced with permission of the American Astronomical Society.2)

SOFIA also presents several other advantages compared with ground-based or space-based (e.g., the European Space Agency's Herschel Space Observatory) IR observatories. For example, unlike space telescopes, SOFIA can be upgraded continually and can be used as a test bed (with conditions similar to those encountered during space flights) for state-of-the-art and high-risk technologies. SOFIA also serves as an excellent ‘training ground’ for a new generation of instrumentalists and experimental astronomers. In addition, the observatory provides the opportunity—via flight participation—for educators to inspire their students with the excitement of scientific research. Another advantage of this airborne platform is the ability to make scientifically interesting observations from different locations around the world. For instance, SOFIA's deployment flexibility enables measurements of transient events, such as stellar occultations that are only visible from certain locations, and observations of objects that are at extreme southern declinations (e.g., the Magellanic Clouds).

In this work,3 we describe the design and operation of the SOFIA platform. We also discuss some of the unique IR science that has been enabled by flying SOFIA within the Earth's stratosphere. Lastly, we provide an overview of the current status of some of SOFIA's science instrument suite. Detailed information regarding the design and capabilities of the SOFIA instrument ensemble is available online.4

The bent-Cassegrain (Nasmyth) design of the SOFIA telescope (supplied by DLR) is illustrated in Figure 3. The telescope is mounted in an open cavity in the aft section of the aircraft fuselage and views the sky through a port-side doorway. We use magnetic torque motors to move the telescope around a spherical bearing, through which the Nasmyth beam passes. The focal plane instruments and the observers (scientists) are located on the pressurized side of the bulkhead (in which the spherical bearing is mounted). This means that researchers and crew can work in a ‘shirtsleeve’ environment during flight. The telescope also has an un-vignetted elevation range of 20–60°. Furthermore, because the cross-elevation travel is only a few degrees, most of the azimuthal telescope movement is achieved by changing the aircraft heading. For this reason, each flight plan is determined according to the list of observation targets. Flights can be as long as 10 hours, with up to 8.5 hours available for observations of science targets.

Figure 3. (a) Schematic diagram of the SOFIA telescope's bent-Cassegrain optical system. (b) A computer-aided design model of the SOFIA telescope assembly. During operation, the telescope and support equipment (shown on the right) are at ambient flight-level pressure, whereas the science instrument and crew (observers) are in the pressurized crew cabin (i.e., to the left of the pressure bulkhead). The telescope (with a mass of 10 metric tons) and the science instrument are supported by a hydrostatic spheric bearing. (Reproduced with permission of the American Astronomical Society.2)

As mentioned above, SOFIA instruments can be repaired, changed, upgraded, modified, and tested aboard the observatory as new technologies are developed. For example, the German Receiver for Astronomy at Terahertz Frequencies (GREAT) instrument (whose team is led by principal investigator Rolf G#usten) has recently been upgraded to provide an array of seven heterodyne on-sky detectors.5 The updated instrument—known as ‘upGREAT’—began operations in 2015 and is already providing exciting science results at a rate that is seven times faster than GREAT. Indeed, to demonstrate the unique and important scientific capabilities of SOFIA, and to provide a publicly available and high-value SOFIA data set, 4.2 hours of Director's Discretionary Time was recently allocated to upGREAT so that a velocity-resolved map of the Horsehead Nebula could be obtained.6 Our resulting ionized carbon (CII) line map at 158μm is shown in Figure 4. The CII line is the dominant cooling line for most of the interstellar medium, and can therefore be used to trace interactions between the high-energy radiation from hot, recently formed stars and the surrounding interstellar medium from which they were born. We estimate that more than 200 hours of observing time would have been required to obtain an equivalent map with Herschel's Heterodyne Instrument for the Far IR (i.e., almost a factor of 50 more than with the SOFIA).

Figure 4. (a) Visible-wavelength image of the Horsehead Nebula. The region mapped during the upGREAT (updated German Receiver for Astronomy at Terahertz Frequencies instrument) SOFIA observations is outlined in red. (b) The upGREAT velocity-resolved ionized carbon (CII) line (at 158μm) intensity map (integrated over a velocity range of 9.5–11.5km/s). Tmb: Brightness temperature.

We are also currently conducting commissioning tests for another updated SOFIA instrument. Our upgraded High-Resolution Airborne Wideband Camera (HAWC) instrument (known as HAWC+) will be used for far-IR imaging and polarimetry. The instrument is capable of producing sensitive maps of linear polarization that will enable, for example, investigations of magnetic field strength and morphology in the interstellar medium of our galaxy and nearby galaxies. The internal design of the instrument is shown in Figure 5, along with a commissioning test image of Jupiter (at a wavelength of 53μm). The HAWC+ instrument includes three 40 × 32 pixel bolometer arrays that allow polarization imaging (in four bands centered near 53, 89, 154, and 214μm) to be achieved at the diffraction limit of the telescope.7 This ‘first light’ image has an angular resolution of about 5arcsec (full width at half-maximum).

Figure 5. (a) Schematic drawing of the updated High-Resolution Airborne Wideband Camera (HAWC+) internal design, showing the location of the cooling system, filters, lenses, and polarimeter hardware. LHe/ADR: Liquid helium/adiabatic demagnetization refrigerator. (b) ‘First light’ HAWC+ image of Jupiter and its four Galilean moons, at a wavelength of 53μm and with an angular resolution of about 5arcsec (full width at half-maximum). Jy: Jansky (unit of spectral flux density).

In summary, we have provided an overview of the SOFIA design and its operations. We have also described some of the most recent upgrades to the observatory's instrument suite. We began SOFIA's Cycle 4 year of operations in February 2016, and in July 2016 we completed a third deployment of the observatory to the southern hemisphere. Three SOFIA instruments—GREAT, the Field-Imaging Far IR Line Spectrometer, and the Faint Object IR Camera for the SOFIA Telescope—were used in this deployment to New Zealand. In the coming year of the SOFIA program, the commissioning of HAWC+ will be completed and will be put into full science operation. We will also commission a second seven-element array for upGREAT. This array will operate at 63μm to allow mapping observations of the atomic oxygen line.

The digitized sky survey shown in Figure 4 was produced at the Space Telescope Science Institute (under US Government grant NAG W-2166). The image is based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK's Schmidt Telescope. The plates were processed into compressed digital form with the permission of these institutions. The SOFIA team is grateful for the use of the US Antarctic Research Program's exisiting hangar, laboratory, and office spaces in Christchurch (for the southern hemisphere deployments). The SOFIA project has also received significant help and support from the Air New Zealand crew and facilities at Christchurch Airport.

Eric E. Becklin, Maureen L. Savage, Erick T. Young
SOFIA Science Center
Universities Space Research Association (USRA)
NASA Ames Research Center
Moffett Field, CA

Eric Becklin is a graduate of the California Institute of Technology. He has more than 50 years of experience in the field of IR observations and instrumentation and has authored more than 300 scientific papers. He holds emeritus status at the University of California, Los Angeles, and serves as USRA's chief scientist for SOFIA.

Dana E. Backman
SOFIA Science Center
SETI Institute
Moffett Field, CA

1. S. D. Lord, A new software tool for computing Earth's atmospheric transmission of near- and far-infrared radiation, Technical Memorandum 103957 NASA, 1992.
2. E. T. Young, E. E. Becklin, P. M. Marcum, T. L. Roellig, J. M. De Buizer, T. L. Herter, R. Güsten, et al., Early science with SOFIA, the Stratospheric Observatory for Infrared Astronomy, Astrophys. J. Lett. 749, p. L17, 2012. doi:10.1088/2041-8205/749/2/L17
3. E. E. Becklin, M. L. Savage, E. T. Young, Stratospheric observatory for infrared astronomy (SOFIA): overview and science results. Presented at SPIE Optics + Photonics 2016.
4. Information on the SOFIA instrument suite. Accessed 12 August 2016.
5. C. Risacher, R. Guesten, J. Stutzki, H.-W. Huebers, A. Bell, C. Buchbender, D. Buechel, et al., The upGREAT 1.9 THz multi-pixel high resolution spectrometer for the SOFIA observatory, arXiv:1607.04239 [astro-ph.IM] , 2016. Accepted for publication in Astron. Astrophys.
7. C. D. Dowell, J. Staguhn, D. A. Harper, T. J. Ames, D. J. Benford, M. Berthoud, N. L. Chapman, et al., HAWC+: a detector, polarimetry, and narrow-band imaging upgrade to SOFIA's far-infrared facility camera, Am. Astron. Soc. Meeting, p. 221, 2013.
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