Lightweight mirror technology using a thin facesheet with active rigid support

The next generation of space telescopes will require primary mirrors that push beyond the current state of technology for mirror fabrication. These mirrors are large, up to 8 meters in diameter, have low mass per unit area, less than 15 kg/rn2 and must maintain diffraction limited performance at cryogenic temperatures. To meet these requirements, we have developed an active mirror that has a thin membrane as the optical surface, which is attached to a stiff lightweight support structure through a set of screw-type actuators. This system allows periodic adjustments with the actuators to maintain the surface figure as measured from star light. The optical surface accuracy and stability are maintained by the active system, so the support structure does not have to be optically stable and can be made using light weight carbon fiber laminates to economically provide stiffness. The key technologies for implementing this technology are now in place. We have performed two critical demonstrations using 2-mm glass membranes -diffractionlimited optical performance of a 0.5-rn diameter mirror and launch survival of a 1-rn diameter mirror. We have also built and tested a prototype actuator that achieves 25 run resolution at cryogenic temperatures. We are now building a 2-rn mirror as a prototype for the Next Generation Space Telescope. This mirror will have mass of only 40 kg, including support structure, actuators, and control electronics. It will be will be actively controlled and interferometrically measured at 35 K.