Reactivation of the active mass damping system for SOFIA to improve image stability

The Stratospheric Observatory for Infrared Astronomy (SOFIA) employs an airborne telescope with a 2.7m primary mirror. The telescope structure is composed of carbon fibre with major parts of steel for the suspension and balancing components. It is exposed to harsh environmental conditions and subject to vibration excitation due to aircraft motions and turbulence from the airflow coming into the telescope cavity. To meet pointing requirements and improve image stability there are ongoing efforts on various components of the telescope system, one of which is the implementation of an Active Mass Damping (AMD) control system: Based on accelerometer signals, reaction mass actuators impose forces onto the support structure to dampen the vibration of optical components. The system has been designed, implemented and preliminary tested in the early years of SOFIA's scientific operation, but concerns about the structural integrity of the primary mirror and new requirements regarding software qualification have prevented the activation and further development for several years. These concerns being addressed, we are now in the process of reactivating the AMD system on the support structure of the primary mirror. Recent ground tests and in-flight jitter measurements indicate that the damping system is very efficient at eliminating the excitation of targeted structural modes of the telescope structure at 40 to 80 Hz and the first bending modes of the primary mirror at 175 Hz, resulting in a significantly improved image quality. This paper presents the analysis of those measurements and discusses options for future development.