As-built Design and Performance of Optical System of the SOFIA Telescope

Abstract

The incoming infrared radiation is reflected by the parabolic primary mirror (PM) with a diameter of 2.7 m, by the subsequently following hyperbolic secondary mirror (SM) with a diameter of 0.352 m and by the plane dichroic tertiary mirror (TM-1) to the focal plane at the Nasmyth focus, also called IR focus. Abstract The Stratospheric Observatory For Infrared Astronomy (SOFIA) is composed of a 2.5-m infrared telescope integrated into a Boeing 747SP aircraft operated at high altitudes. The central opto-mechanical subsystem of the SOFIA Cassegrain telescope is the primary mirror assembly with its extremely stiff mirror support structure and the 2.7-m primary mirror. The mirror is a monolithic element of Zerodur with a milled honeycomb structure on the backside. Despite of its size it has a mass of approx. 885 kg only. The mirror support 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 an isostatic but stiff mounting of the mirror in the support structure (first natural Eigenfrequency around 70 Hz). The secondary mirror is a SiC mirror suitable for 20-Hz-rectangle chopper actuation enabling efficient background suppression especially for far infrared observations. The tertiary mirror providing the Nasmyth focus is implemented as a dichroic beam splitter transmitting the visible part of the incoming radiation for tracking purposes. The paper describes the hardware design of the optical system, the tested as-built performance of the subassemblies as well as the predicted overall image performance from optical ray-tracing simulations based on the interferometric measurements of the optical subassemblies. As well the status of the currently ongoing integration of the optical system into the aircraft will be presented. IR Focus TM1/2 SM