Abstract
The Stratospheric Observatory for Infrared Astronomy (SOFIA) consists of a 2.7[Formula: see text]m 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 7[Formula: see text]ms providing about 3.3% higher efficiency for observations with 5[Formula: see text]Hz chopping.
Highlights
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory that consists of a 2.7 m diameter infrared telescope, mounted in the aft fuselage of a modied Boeing 747 SP aircraft (Krabbe, 2000)
Therst dominant unwanted resonance that results in mirror jitter is at about 300 Hz. This resonance is addressed by the Tilt chopping mechanism (TCM) controller, restraining the unwanted mirror motion at the expense of mirror steering performance. Reducing this resonance on the structural level would allow for a wider actuation bandwidth, which would lead to a faster transition between the two chop positions and reduce the image jitter introduced by external disturbances acting on the active mechanism itself
A Finite Element (FE) model for the numerical modal analysis of the Secondary Mirror Assembly (SMA) was prepared by Lamparsky (2013) and optimized by Peter (2015)
Summary
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory that consists of a 2.7 m diameter infrared telescope, mounted in the aft fuselage of a modied Boeing 747 SP aircraft (Krabbe, 2000). To eliminate any load induction to the telescope structure when chopping, the lever arm assembly is tilting a ringshaped reaction mass, thecompensation ring', in the mirror's opposite direction. This ring is mounted to the three lever arms by a set of °exures. This resonance is addressed by the TCM controller, restraining the unwanted mirror motion at the expense of mirror steering performance Reducing this resonance on the structural level would allow for a wider actuation bandwidth, which would lead to a faster transition between the two chop positions and reduce the image jitter introduced by external disturbances acting on the active mechanism itself (wind loads and telescope vibrations)
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