A one meter diameter prototype lightweight composite mirror is being developed. The mandrel on which the composite mirror will be laid up is an ultra low expansion quartz glass, TSG, whose thermal expansion coefficient, 10~(-7)/°C or less, is similar to that for the composite material. A precise layup procedure for the composite material is required to obtain this coefficient of expansion. The mandrel surface will be superpolished to 6-8 A rms or better, resulting in ten times less scattered light in the visible region than displayed by a typical astronomical mirror. It has been shown experimentally that mandrel microroughnesses of this order can be successfully transferred to a replicated composite mirror faceplate. The composite faceplate is one to three millimeters thick, very tough, and unlike thin glass adaptive optic faceplates does not fracture easily. Actuators designed for atmospheric correction of the thinner adaptive optic faceplates have a response time of 1/2 msec, which is fast enough to correct for all atmospheric distortion. The thicker faceplates are used for active mirrors and are mounted differently. Controlled by stepper motors they can be used for computer controlled remote focusing over long distances or fine tuning of mirror tip-tilt as well as for gravitational sag of other mirrors in the optical train. All mirror actuators operate in the 30-70 v range.
展开▼
