A new finite element model of the SOFIA primary mirror cell to investigate dynamical behavior

Abstract

The telescope structure of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is subject to vibration excitation due to aircraft motions and turbulence from the airflow coming into the telescope cavity. A proper understanding of the dynamical behavior of the telescope structure under operational loads is crucial for pointing control and measures against higher order optical aberrations. During design and construction a Finite Element model of the telescope assembly has been created in order to assess the structural integrity and the early performance. This legacy model used conservative assumptions and had a coarse approach on the approximation of some structural features. We present an updated Finite Element model of the SOFIA Primary Mirror Assembly, which represents support members as well as the primary mirror itself in greater detail, in order to support ongoing development for performance optimization. An iterative approach employing structural optimization was used to tune the model in order to fit modal parameters of the Primary Mirror Assembly which were measured in a test campaign prior to integration into the full telescope structure. The updated and tuned model is used to calculate deformations due to gravity, thermal loads and dynamic excitation. These deformations serve as input for ray-tracing analyses to investigate alterations in the light path in order to evaluate pointing errors and higher order optical aberrations.