A lightweight, cryogenically capable, scalable, deformable mirror has been developed for space telescopes. This innovation makes use of polymer-based membrane mirror technology to enable large-aperture mirrors that can be easily launched and deployed. The key component of this innovation is a lightweight, large-stroke, cryogenic actuator array that combines the high degree of mirror figure control needed with a large actuator influence function. The latter aspect of the innovation allows membrane mirror figure correction with a relatively low actuator density, preserving the lightweight attributes of the system.
The principal components of this technology are lightweight, low-profile, high-stroke, cryogenic-capable piezoelectric actuators based on PMN-PT (piezoelectric lead magnesium niobatelead titanate) single-crystal configured in a flextensional actuator format; highquality, low-thermal-expansion polymer membrane mirror materials developed by NeXolve; and electrostatic coupling between the membrane mirror and the piezoelectric actuator assembly to minimize problems such as actuator printthrough. PMN-PT single-crystal material provides a piezoelectric driver that delivers appreciable strain from above room temperature to less than 20 K. The combination of a polymer membrane material for the mirror and the flextensional actuator design results in a very lightweight structure with a large range of aberration correction.
The membrane mirror is a low-stiffness component that requires relatively low actuator force. The flextensional actuator design is a low-force, high-displacement (>400 microns), lightweight piezoelectric positioning technology. The combination of the two results in a much lighter-weight structure with higher correction range than can be achieved with conventional piston-style actuators and glass face sheets. To combat actuator print-through and to lessen actuator density, a hybrid piezoelectricelectrostatic actuation approach was developed. The actuators push on an electrode plate held at a high voltage. The plate is coupled to the mirror through the electrostatic field established by the applied voltage, but does not make direct mechanical contact with the mirror. As the actuators move the electrode plate, the mirror is stretched or relaxed as needed. This allows a high degree of figure control with a relatively small actuator density. Control can be further enhanced by including multiple actuators for each electrode plate, allowing both piston and tip/tilt motion.