The trend in future space telescopes points toward increased primary mirror diameter, which improves resolution
and sensitivity. However, given the constraints on mass and volume deliverable to orbit by current launch
vehicles, creative design solutions are needed to enable increased mirror size while keeping mass and volume
within acceptable limits. Lightweight, segmented, rib-stiffened, actively controlled primary mirrors have emerged
as a potential solution. Embedded surface-parallel actuators can be used to change the mirror prescription onorbit,
lowering mirror mass overall by enabling lighter substrate materials such as silicon carbide (SiC) and
relaxing manufacturing constraints. However, the discrete nature of the actuators causes high spatial frequency
residual errors when commanding low-order prescription changes. A parameterized finite element model is used
to simulate actuator-induced residual error and investigate design solutions that mitigate this error source.
Judicious specification of mirror substrate geometry and actuator length is shown to reduce actuator-induced
residual while keeping areal density constant. Specifically, a sinusoidally-varying rib shaping function is found to
increase actuator influence functions and decrease residual. Likewise, longer actuators are found to offer reduced
residual. Other options for geometric shaping are discussed, such as rib-to-facesheet blending and the use of two
dimensional patch actuators.