Solving a Performance Limiting Resonance Frequency Problem of the SOFIA  Secondary Mirror Mechanism by Structural Modifications  Yannick Lammen*,†, Andreas Reinacher*,† and Alfred Krabbe*  Abstract  The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.7-m infrared telescope built into a  Boeing 747 SP. It reached its full operational capability in 2014 and takes off about three to four times a  week to explore the infrared sky from stratospheric altitudes above most of the atmosphere’s water vapor  content. Since its installation in 2002, the active Secondary Mirror Mechanism (SMM) has proven to be a  reliable part of the telescope assembly. However, its performance is limited by a strong structural resonance  at a frequency of about 300 Hz. Solving this resonance on the hardware level would lead to a wider  actuation bandwidth and therefore a faster transition time for infrared chopping and pointing corrections.  Based on finite element simulations, a ring-shaped reaction mass made of aluminum has been identified  as the main cause of this mode. Concepts have been developed to eliminate the resonance by changing  the mass distribution along the ring shape and by implementing a parallel kinematic suspension. An endto-end simulation including the mechanical finite element model and a model of the control algorithm was  used to predict the final system performance of the developed concepts. Based on the simulation results,  a prototype of the new reaction mass was designed and manufactured. An extreme mass redistribution  along the ring shape by combining tungsten and AlSiC ring-segments shows the best results. The new ring  was integrated and thoroughly tested on a mockup of the SMM confirming the predicted performance  improvement. The closed-loop actuation bandwidth is improved by 80%. Based on this prototype, two flight  units will be manufactured for future SOFIA science missions.   Introduction  After reaching its full operational capability in 2014, the Stratospheric Observatory For Infrared Astronomy  (SOFIA) takes off several times a week to explore the universe. This, today’s largest airborne observatory,  consists of a 2.7-m-diameter infrared telescope mounted in the aft fuselage of a modified Boeing 747SP  aircraft. Maximizing the scientific output requires successive optimizations of the telescope performance  while not interfering with science.  One of the most complex systems of the observatory is the Secondary Mirror Mechanism (SMM) providing  fast mirror steering capability for image stabilization and infrared square wave chopping. The SMM is  mounted 446 mm before the focus of the primary mirror, close to the cavity opening and exposed to harsh  environmental conditions (see Figure 1). After more than 10 years of use and 400 flights, this mechanism,  designed and manufactured by CSEM in Switzerland, proves to be a reliable part of the observatory.  However, since its integration in 2002 the performance of the SMM is limited by a strong structural  resonance at a frequency of 300 Hz. For more information about SOFIA and the SMM refer to [1], [2] and  [6].  *Deutsches SOFIA Institut, University of Stuttgart, Stuttgart, Germany † SOFIA Airborne Systems Operations Center, NASA Armstrong Flight Research Center, Edwards CA Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018 NASA/CP—2018-219887 165