Next Generation Space Telescope. II: Proposed Pointing Control System

Abstract

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.