The advancing requirements of instrumentation in astronomy has 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 3-degree of freedom (DoF) parallel manipulator of the type described by Lee and Shah (1988a, 1988b). The workspace of this manipulator is examined for two types of actuator mounting geometries. In the first case, pin joint connections are placed at the base and ball joints at the end-effector. In the second case, the joints are reversed. Following a general investigation, the results are specialized to reflect the behavior of small motions, as active surfaces require only small adjustments. Finally, the effects of the kinematics on the performance of an example surface are determined. The analysis reveals that placing the ball joint connections at the base provides a passive correction of uncontrolled states in the surface.

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