Image quality prediction for the SOFIA telescope using actual mirror data

The airborne Stratospheric Observatory for Infrared Astronomy SOFIA has a 2.7-m Cassegrain telescope with Nasmyth focus. The optical elements of the telescope are a 2.7-m diameter Zerodur parabolic primary mirror, mounted in a CFRP mirror support structure, a 0.35-m diameter convex silicon carbide secondary mirror, mounted on a chopping mechanism, and a plane dichroic tertiary mirror element. The nominal telescope focal length is 49.14 m. There are several distortion contributions to the total image quality. Among these are alignment errors, fabrication imperfections stemming from the manufacturing and polishing processes, static and dynamic mirror surface deformations caused by the mounting forces due to gravity and temperature effects and forces induced by aircraft vibra-tions and aerodynamic effects, and diffraction effects. The paper presents simulations for the image quality using wave-front performance data actually measured with the finished mirrors, where the primary was even mounted in its support structure. The influence of the static distortion contributions is also quantified.