The Primary Mirror of the Keck Observatory Telescope is made up of an array of 36 hexagonal mirror segments underactive control. The measurement of the relative orientations of the mirror segments is fundamental to their control. The mechanical and electronic design of the sensors used to measure these relative positions is described along with the performance of the sensors under a variety of tests. In use, the sensors will measure relative positions with a resolution of a few nanometers. This

more »

... resolution and the low noise, drift and thermal sensitivity of the sensors are adequate to stabilize the primary mirror figure to the precision required for optical and infrared astronomy.