2.1 The Observatory

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' at a frequency ≤ 10 Hz and up to ± 10' 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 the 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.

Eight instruments, covering a wide range of wavelengths and resolving powers, are available for use on SOFIA. Three of the instruments are Facility-class Science Instruments (FSIs), which will be maintained and operated by the Science Mission Operations (SMO) staff.

Two instruments are Principal Investigator-class Science Instruments (PSIs), which will be maintained and operated by the Instrument Principal Investigator (PI) teams.

One instrument is a Special Purpose, Principal Investigator-class Science Instrument (SSI).

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