Position actuators for the primary mirror of the W. M. Keck telescope

The pistons and tilts of the 36 segments of the Keck Telescope primary mirror are under active control. The mechanical and electronic designs of the actuators used to achieve this control are described along with the performance of the actuators under a variety of tests. In use, the actuators will move in 4-nm increments. This resolution and the accuracy of the actuator moves are adequate for stabilizing the figure of the primary mirror to the precision required for optical and infrared astronomy.