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11 August 2008SIM Planet Quest Lite Interferometer Guide 2 Telescope pointing control system
The Space Interferometry Mission - Planet Quest Lite (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.