Increasing the SOFIA Secondary Mirror Mechanism’s Fast Steering Capability by Identification of a Structural Resonance and Its Subsequent Elimination Through Mass Re-Distribution

The Stratospheric Observatory for Infrared Astronomy (SOFIA) consists of a 2.7m infrared telescope integrated into a Boeing 747 SP. One of the most complex subsystems of the observatory is the secondary mirror assembly (SMA). This active steering mechanism is used for image stabilization and infrared chopping. Since its integration in 2002, the performance of the mechanism is limited by a structural resonance. Based on Finite Element (FE) simulations and experimental modal surveys, a ring shaped reaction mass was identified to be the causing element of this structural mode. Attenuating the resonance on the hardware level would result in a larger actuation bandwidth for faster chopping and image stabilization. Concentrating mass at the suspension points while keeping the inertia of the ring structure is expected to take strain energy out of the mode. An end-to-end simulation, including a FE model of the mechanism and a controller model was set up to predict the in-flight performance of this concept. A segmented ring made from tungsten and AlSiC (i.e. strong mass redistribution) mounted on the original suspension was selected for the design of a prototype. The prototype was manufactured and thoroughly tested on a full-scale mockup of the mechanism confirming the predicted performance. An actuation bandwidth improvement of 80% was achieved. The settling time for infrared chopping was reduced from 10 to 7ms providing about 3.3% higher efficiency for observations with 5Hz chopping.