The tracking subsystem of the SOFIA telescope

The Tracking Subsystem of the SOFIA telescope consists of three high performance imagers and a dedicated tracking control unit. There are two boresighted imagers for target acquisition and tracking, one with a wide (6 degrees) and one with a fine (70 arcmin) field-of-view, and one main-telescope-optics sharing imager with a narrow field-of-view (8 arcmin) for high performance tracking. From the recorded stellar images, tracking error signals are generated by the tracker controller. The tracker controller has several features to support various tracking schemes such as tracking the telescope as an inertial platform, on-axis/offset tracking, and limb tracking. The tracker has three modes, i.e. positioning, tracking and "override". Special features are the handling of so-called areas-of-interest in the inertial reference frame and the external imager synchronization. The paper presents the design and functional / operational performance of the imagers and the tracking control unit.