while the resulting optical layout has an f-number of 19.7. A flat, tertiary, dichroic mirror was used to deflect the infrared part of the beam to the Nasmyth focus where it can be analyzed. An optical mirror located behind the tertiary mirror was used for a camera guidance system. The telescope looks out of a large door in the port side of the fuselage near the airplane's tail, and initially carried nine instruments for infrared astronomy at wavelengths from 1–655 micrometres (μm) and high-speed optical astronomy at wavelengths from 0.3 to 1.1 μm. The main instruments were the FLITECAM, a near-infrared camera covering 1–5 μm; FORCAST, covering the mid-infrared range of 5–40 μm; and HAWC, which spans the far-infrared in the range 42–210 μm. The other four instruments include an optical photometer and infrared spectrometers with various spectral ranges. SOFIA's telescope was by far the largest ever to be placed in an aircraft. For each mission one interchangeable science instrument will be attached to the telescope. Two groups of general-purpose instruments were available. In addition, an investigator can also design and build a special purpose instrument. On April 17, 2012, two upgrades to HAWC were selected by NASA to increase the field of view with new transition edge sensor bolometer detector arrays and to add the capability of measuring the polarization of dust emission from celestial sources. The open cavity housing the telescope will be exposed to high-speed turbulent winds. In addition, the vibrations and motions of the aircraft introduce observing difficulties. The telescope was designed to be very lightweight, with a honeycomb shape milled into the back of the mirror and polymer composite material used for the telescope assembly. The mount includes a system of bearings in pressurized oil to isolate the instrument from vibration. Tracking was achieved through a system of gyroscopes, high-speed cameras, and magnetic torque motors to compensate for motion, including vibrations from airflow and the aircraft engines. The telescope cabin must be cooled prior to aircraft takeoff to ensure the telescope matches the external temperature to prevent thermally-induced shape changes. Prior to landing the compartment is flooded with nitrogen gas to prevent condensation of moisture on the chilled optics and instruments. DLR was responsible for the entire telescope assembly and design, along with two of the nine scientific instruments used with the telescope; NASA was responsible for the aircraft. The manufacturing of the telescope was