A system to measure image size and motion has been designed, constructed and put into operation at the 4-meter Mayall telescope at Kitt Peak National Observatory (KPNO). The present system configuration consists of three elements. A 128 multiplied by 128 pixel CID camera is mounted at the south port of the Cassegrain instrument rotator. The CID camera is fed, under software control, by a movable mirror in the instrument rotator. Focusing is achieved by determining the point of zero-image moti...