Next generation space telescope. II: Proposed pointing control system

The pointing control system of the Hubble Space Telescope (HST) represents the current state of the art for the precision control of a large spacecraft. The proposed Next Generation Space Telescope (NGST) will require an order-of-magnitude increase in pointing resolution over that of HST. The use of active optics in the form of a steerable secondary mirror has been proposed for NGST to satisfy these requirements. The primary motivation for this study was to demonstrate the feasibility of satisfying the pointing-stability requirements by sensing the guide-star position and steering the optical path of the telescope with the active secondary mirror. To study the requirements of the control system, a two-degree-of-freedom model that retains the rigid-body mode of the telescope as well as its first oscillatory mode was constructed. The corresponding optimal control law was developed and implemented in a discrete manner to examine the behavior of the system subject to typical spacecraft excitations.