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NSO/Sac Peak continuous-face-plate adaptive mirror

The National Solar Observatory is constructing a continuous-face-plate mirror with 61 actuators. The mirror, which has a clear aperture of 218 mm, features a detachable face plate and replaceable actuators that are servoed to maintain a position measured by capacitors which are within the actuators themselves. The actuators are 20-mm diameter and are placed on 32-mm centers. Each has a range of 6.4 microns for a voltage swing of +/- 175 volts. The servos have a bandwidth of 1 KHz (-3 db). In order to couple the face plate to the actuators, each actuator has a rare-earth magnet on its end that attracts a steel button cemented to the 3-mm-thick glass faceplate. The mirror is a contender for those systems needing a relatively large adaptive mirror with relatively few actuators, for example, to replace the secondary mirror of a Cassegrain telescope. At Sacramento Peak the mirror will be used with the 76-cm aperture Vacuum Tower Telescope to observe small details on the sun.