SIM Planet Quest Lite Interferometer Guide 2 Telescope pointing control system

Abstract

The Space Interferometry Mission - Planet Quest (SIM-PQL) Guide 2 telescope system is currently being developed at JPL. The SIM-PQL is a new mission concept to perform micro-arcsecond narrow-angle astrometry to search approximately 50 nearby stars for Earth-like planets, and to perform global astrometry with an accuracy of six micro-arcseconds in position and parallax. The novel cost and mass reducing Lite concept includes reduction in interferometer baselines and replacing the second guide interferometer (Guide 2) with a telescopic system. The resulting simplification still allows meeting most science goals without significant performance degradation. The Guide 2 telescope employs a nulling pointing control system that utilizes a Fast Steering Mirror (FSM) as an actuator and a star tracking CCD camera as a control sensor. Under the nulling closed loop control, the modulated attitude motion of the instrument is picked off by a metrology FSM tip-tilt sensor (AMET). The Guide 2 pointing control system requires mili-arcsecond class fine pointing, maintenance of low jitter and thermal stability and a sub-nanometer class metrology system that ties the sensor bench to rest of the instrument. This paper presents the Guide 2 telescope pointing control system design and resulting performance estimates. The pointing control requirements are first stated followed by the descriptions of the system architecture, algorithm design and simulation results. Concept and algorithm validation is conducted on a workstation-based simulation testbed, specifically developed to capture critical sensor/actuator behavior and environmental disturbances.