Under an agreement with NASA,
DLR (the German Aerospace Center)
will supply the telescope for the SOFIA observatory as well as operation
support in exchange for observation time aboard the SOFIA.
The SOFIA telescope is designed as a Cassegrain system
with two Nasmyth foci, the nominal IR focus and an additional visible
light focus for guiding. IR light is reflected off the upper tertiary
mirror, which has a dichroic coating. The dichroic can be replaced
with a fully reflective mirror if guiding with the focal plane guider
is not required.
It has a parabolic 2.7m primary mirror and a hyperbolic
secondary mirror. The secondary mirror is attached to a chopping
mechanism providing chop amplitudes of up to ± 5 arcmin at
chop frequencies between 0 and 20 Hz, programmable by either a user
supplied analogue or TTL curve or by the telescope control electronics.
A flat tertiary mirror reflects the beam into the infrared Nasmyth
focus, 300mm behind the instrument flange. If the tertiary is replaced
by a dichroic mirror, the transmitted optical light is reflected
by a second tertiary 289.2mm behind the dichroic and sent to the
visible Nasmyth focus. There it is fed into a focal plane guiding
camera system (FPI). Independent of the FPI there are two other
imaging and guiding cameras available: the wide field imager (WFI)
and the fine field imager (FFI). Both of these cameras are attached
to the front ring of the telescope.