Herschel Pointing Accuracy Improvement Hitting the bullseye from 3234,03 meters distance - History of the Herschel Attitude Control System Pointing Accuracy Improvement

For the Herschel Space Observatory, launched on May 14, 2009, as part of the ESA Horizon 2000 Science Program, one critical aspect is the pointing accuracy as provided by the Attitude Control and Monitoring System (ACMS). After verification of the in-orbit performance of the ACMS during the Service Module Commissioning Phase, which revealed and corrected for interference from the thermal control of the Gyro Unit (Scalable Inertial Reference Unit), the pointing accuracy was monitored and improved during various campaigns. This paper provides a summary of the measures taken to enhance the performance of Herschel’s Attitude Control System beyond manufacturer specifications by eliminating interacting factors such as the identification and removal of “warm” pixels on the Star Tracker CCD, adaption of the focal length in the Star Tracker on-board software stepwise in one, two and then three dimensions and a clean-up of the entries in the Star Tracker on-board Star Catalogue. Herschel is now observing with accuracy in the order below 1 arcsec for the Absolute Pointing Error.