The SOFIA Science Instruments

The SOFIA observatory had a wide range of scientific instruments (SI) at its disposal, which were typically installed on the telescope's instrument flange for two to three weeks at a time. This allowed spectroscopic, photometric, and polarimetric measurements to be carried out. The scientific observations determined which of the instruments had to be used. The flight planners' task was to combine different observations with the same instrument at the right time of year. This is because not every astronomical object is visible throughout the year; some can only be seen from the northern or southern hemisphere of the Earth.
The FPI+ (Focal Plane Imager) tracking camera occupies a special position among the instruments. It used the telescope optics and was able to perform photometry with high temporal resolution during the observation of stellar occultations. In addition, it was used as a telescope guide camera for all observations.
The telescope also had two additional cameras, the Wide Field Imager (WFI, focal length 136 mm, field of view 6 x 6 degrees) and the Fine Field Imager (FFI, focal length 733 mm, field of view 67 x 67 arcmin). These were attached to the front ring of the telescope and were primarily used for orientation in the sky and for recognizing star patterns by the telescope operators. WFI and FFI are not classified as scientific instruments.

The most important scientific instruments used by SOFIA, whose data is also being processed as part of the SDC project, were (in alphabetical order):

High-resolution Echelon/Echelle spectrograph for mid-infrared (4.5–28.3 mm) and high spectral resolution up to R~10⁵.

Imaging grating spectrometer with two channels in the far-infrared wavelength range (51–120 mm, 115–200 mm) and medium resolution of R~600–2,000).

Dual-channel camera in the mid-infrared range (5-40 mm), which also enabled low-resolution spectroscopy (R~200) using four grisms (a combination of prism and diffraction grating).

Guide camera of the telescope, photometric scientific instrument, primarily used for stellar occultations and as an analysis instrument for monitoring the observatory's performance.

Heterodyne receiver, which enabled extremely high-resolution spectroscopy (R~10⁸) in different frequency bands of the far-infrared spectrum in two different configurations. The data reduction pipeline for the GREAT instrument is entirely under the responsibility of the University of Cologne and is processed exclusively there. In particular, it is not part of the Redux pipeline.

Dual-channel far-infrared camera with broadband filters that are sensitive in the wavelength range between 50 and 240 mm and can also measure polarization. This made it possible to measure and visualize magnetic fields in space.

The HIPO (High-Speed Imaging Photometer for Occultations) instrument was used to make a number of observations during the initial phase of the project. HIPO was also an important test instrument during the development phase of SOFIA and was the only SOFIA instrument to successfully complete the FAA airworthiness process (NASA later took over this certification itself). It has been used several times to observe occultations, most recently in 2017 during the occultation by Neptune's satellite Triton.

The FLITECAM (First Light Infrared Test Experiment Camera) instrument was, as its name suggests, designed as a near-infrared camera for testing purposes during the commissioning of the observatory. In addition, the instrument also had a spectroscopic operating mode in the near-infrared, which made it a fully-fledged scientific instrument of SOFIA (Facility Class SI). A special feature was that FLITECAM could be mounted and operated together with HIPO on SOFIA. For this purpose, a dichroic beam splitter was mounted in front of both instruments. The resulting combination was called FLIPO and was used during commissioning and subsequently also in regular missions, especially for star occultations.

Due to the decision made in 2016 to focus SOFIA on FIR observation, the Triton occultation was the last mission for HIPO and FLITECAM. The number of observations carried out and the resulting amount of data is small overall. Therefore, these instruments are not included in the SDC's work. However, the available data can be accessed without restriction from IRSA.

The following graph provides an overview of the wavelength ranges and spectral resolution of the various SOFIA instruments. Each of the instruments was designed and optimized to answer specific scientific questions.