Integrated modeling and dynamics simulation for the Next Generation Space Telescope (NGST)

NGST represents a challenging problem From the point of view of maintaining a milli-arcsecond level pointing accuracy and diffraction limited wavefront performance in the presence of dynamic onboard disturbances during science observations in a cryogenic environment. A Dynamics-Optics-Controls-Structures framework is being developed in support of the NGST dynamics and controls modeling program. First, an integrated model comprising multiple disturbance sources, structures, optics and control systems was developed in order to predict the expected dynamic wavefront error and line-of-sight jitter. A disturbance analysis was carried out using frequency domain and Lyapunov techniques. An indexing technique was subsequently used to analyze the sensor-actuator topology of the system. A sensitivity analysis revealed which modal parameters contribute significantly to the root-mean-square errors. The critical frequencies were found to be in the range from 5-50 Hz. This information was subsequently used to recommend performance improvements including stiffening the secondary mirror tower, isolating reaction wheels and adding passive damping treatments. The combination of improvements was implemented numerically, resulting in good performance with sufficient design margins. Isoperformance analysis was introduced as a means of trading system parameters, while holding the performance constant. This research is motivated by the fact that it will not be possible to lest the fully deployed observatory in a Ig environment before launch.