An end-to-end simulation to predict the in-flight performance improvement of a modified SOFIA secondary mirror mechanism

One of the most complex systems of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is the Secondary Mirror Assembly (SMA) providing fast mirror steering capability for image stabilization and infrared square wave chopping. Since its integration in 2002 the performance of the SMM is limited by a strong structural resonance caused by the deformation of a ring-shaped reaction mass. Constraining this resonance would not only lead to a wider actuation bandwidth and therefore a faster transition between the chop positions but also reduce the image jitter introduced by external disturbances acting on the active mechanism itself. Concepts have been developed to attenuate this resonance by structural modifications on the hardware level. To predict the later in-flight performance of these concepts an end-to-end simulation has been setup. The design changes are implemented into a finite element model of the SMA to compute the open loop system response of the mechanism. Subsequently the new dynamic system behavior is implemented into a controller model to simulate the closed loop controlled SMA. Next to the new steering bandwidth, the disturbance rejection capability is analyzed by applying a white noise excitation simulating wind loads and process noise. Moreover, the transition time between the chop positions is determined by applying a square wave input signal to the simulation.