The optical system of the SOFIA telescope

The Stratospheric Observatory For Infrared Astronomy (SOFIA) houses a 2.5-m infrared telescope in a Boeing 747SP aircraft. It will be operated at high altitudes above the atmospheric water vapor. The telescope is of Cassegrain type in a so-called Nasmyth configuration with a VIS and an IR focus. The 2.7-m primary mirror is a monolithic element of Zerodur with a milled honeycomb structure on the backside. Despite of its size, it has a high stiffness (approx. 160 Hz 1st natural frequency) and a mass of approx. 885 kg only. The mirror support structure is a lightweight structure made from CFRP panels and profiles, bonded and riveted together with metallic inserts and joints. The mirror mounting by dedicated flexures provides a very stiff but nevertheless isostatic mounting of the mirror in the support structure (first natural frequency around 70 Hz). The secondary mirror is made from SiC with a very high 1st natural frequency of approx. 2 kHz allowing noise-suppressing chopper operation without image distortions. The tertiary mirror assembly is implemented as a dichroic beam splitter providing the IR Nasmyth focus for the scientific instruments and as a fully reflective mirror providing the VIS Nasmyth focus for tracking purposes. The paper describes the optical system with its subassemblies, their tested as-built performance as well as the predicted extrapolated overall image performance. The integration of the primary mirror assembly into the aircraft will be shown. The further integration and alignment steps, planned for summer 2004, will be explained.