1.1 SOFIA and Its Instruments

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1.1   SOFIA and Its Instruments

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a telescope with an effective diameter of 2.5 meters, carried aboard a Boeing 747-SP aircraft. It is the successor to the smaller Kuiper Airborne Observatory, which was operated by NASA from 1974 to 1996. The observing altitudes for SOFIA are between 37,000 and 45,000 feet, above 99% of the water vapor in the Earth's atmosphere. The telescope was designed to allow imaging and spectroscopic observations from 0.3 to 1600 μm, with specific capabilities dependent on an evolving science instrument suite, making it one of the premier facilities for astronomical observations at infrared and sub-millimeter wavelengths. The present instrument suite provides coverage from 0.3–612 μm with additional capabilities currently in development.

The telescope for SOFIA was supplied by the DLR as the major part of the German contribution to the observatory. It is a bent Cassegrain with 2.7 m parabolic primary mirror (2.5 m effective aperture) and a 0.35 m diameter hyperbolic chopping secondary mirror. The telescope is mounted in an open cavity in the aft section of the aircraft and views the sky through a port-side door. The telescope is articulated by magnetic torque motors around a spherical bearing through which the Nasmyth beam passes. The unvignetted elevation range of the telescope is 20°–60°. The cross-elevation travel is only ± 3° and, therefore, most of the azimuthal telescope movement required for tracking is provided by steering the airplane. Thus, the flight plan is determined by the list of targets to be observed during each flight.

The telescope feeds two f/19.6 Nasmyth foci, an IR focus for the science instruments and a visible light focus for guiding, using a dichroic and an aluminum flat. The secondary mirror is designed to chop at amplitudes of up to ± 5 arcmin at a frequency ≤ 10 Hz and up to ± 10 arcmin at a rate of ≤ 2 Hz. The visible beam is fed into the Focal Plane Imager (FPI+), which is an optical focal plane guiding camera. Independent of FPI+ are two other optical imaging and guiding cameras, the Wide Field Imager (WFI) and the Fine Field Imager (FFI), both of which are installed on the front ring of the telescope.

Six instruments, covering a wide range of wavelengths and resolving powers as shown in Figure 1.1-1, are available for use on SOFIA. Two of the instruments are Facility-class Science Instruments (FSIs), which will be maintained and operated by the Science Mission Operations (SMO) staff.

FIFI-LS: Far Infrared Field-Imaging Line Spectrometer
An integral-field far-infrared spectrometer
 
FORCAST: Faint Object InfraRed Camera for the SOFIA Telescope
A focal plane CCD imagerd-IR camera (including its grism modes) (FSI)
 
FPI+: Focal Plane Imager
A focal plane CCD imager
 
HAWC+: High-resolution Airborne Wideband Camera + Polarimeter
A far-IR camera and polarimeter (FSI)
 
Two instruments are Principal Investigator-class Science Instruments (PSIs), which will be maintained and operated by the Instrument Principal Investigator (PI) teams.
 
EXES: Echelon-Cross-Echelle Spectrograph
A mid-infrared high-resolution spectrograph (PSI)
 
GREAT: German Receiver for Astronomy at Terahertz Frequencies
A heterodyne spectrometer, including the seven-beam receiver array upGREAT (PSI)
 
Figure 1.1-1.
SOFIA instruments resolving power vs wavelength plot

Figure 1.1-1. The resolving power ranges of the SOFIA instrument suite.

The instrument capabilities, the available modes, and the resulting performance specifications of the telescope are described in later sections. For the purpose of this document, configuration refers to the setup of the telescope and instrument whereas mode refers to observational techniques employed during operations. Common combinations of configurations and modes are represented as selectable options within the Unified SOFIA Proposal and observation Tool (USPOT) via individual Astronomical Observation Templates (AOTs). Please note that this naming convention may not necessarily be employed uniformly in external resources for the instruments, i.e. in websites or documentation not managed by the SOFIA team.

Most of the observing time on SOFIA is open to the international astronomical community via Guest Observer (GO) proposal calls, which are issued on a yearly basis. The first of these proposal calls was for Early Science, for which observations were obtained in a series of flights from May–July, September, and November 2011. The first open call for proposals, Cycle 1, covered the period from late 2012 to the end of 2013. The current proposal call is for Cycle 7 observations, solicited by USRA on behalf of NASA. The observations will take place during a series of Science Flight Campaigns, each of which will focus on a single instrument configuration, over the duration of the cycle. The campaigns will be interspersed with aircraft maintenance and instrument commissioning. A single Southern Hemisphere observing series is under consideration for the Cycle 7 time period during the southern winter.

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