Kinematic passive correction of errors in active surface telescopes

Abstract

The advancing requirements of instrumentation in astronomy have led to the development of telescopes with segmented active surfaces. In such telescopes, movable segments are controlled together to maintain the correct figure of the reflecting surface. This paper investigates telescope active surfaces by representing each movable segment as a three-degree-of-freedom parallel manipulator. In this representation, the performance of the active surface can be understood by investigating the small-motion workspace of the manipulator. The workspace is examined for two types of actuator mounting geometry which use only revolute and spherical (ball) joints. In the first geometry, the pin joints are located at the base and ball joints are used to connect to the segment. In the second case, the joint arrangement is reversed. Following the workspace investigation, the effects of the kinematics on the performance of an example surface are determined. The analysis reveals that placing all of the ball joint connections at the base provides a passive correction of uncontrolled degrees of freedom in the active surface, significantly improving telescope performance.