A new backup secondary mirror for SOFIA

Abstract

The telescope of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is a Cassegrain design with a convex, hyperbolic secondary mirror. It is 352 mm in diameter, was made from silicon carbide and weighs only 1.9 kg. As this material is brittle, and the secondary mirror is indispensable to observations with SOFIA, a backup with the same mass and moments of inertia was made of aluminium in 2004. This mirror, however, allows diffraction-limited observations only above 20 μm and it produces double peaked images. In this paper we discuss the requirements for a new backup secondary mirror that can be employed also at near-infrared and even visible wavelengths and describe the most important aspects of the manufacturing process. The starting point of our analysis was a high-precision measurement of the surface properties of the existing aluminium secondary mirror, using the NANOMEFOS technique, which was recently developed by TNO in Delft, the Netherlands. With the exact shape of the mirror as input for a Zemax model we could reproduce the results of actual measurements of its optical performance that had been carried out on SOFIA in 2004. Based on these findings we determined then the specifications to be fulfilled by a new backup secondary mirror in order to meet the requirements on improved optical performance. Finally, we discuss the dynamic deformation of the aluminium mirror during chopping motions.