TMT is a complex system-of-systems. As a result, the telescope controls are responsible for over 30,000 input/output channels and nearly 12,000 controlled degrees of freedom. This includes controlling the pointing and tracking of the mount, the position, tilt and active optics shape control of the primary, secondary and tertiary mirrors, the alignment and phasing system, the wavefront sensors built into the instruments, and coordinating the motions of the enclosure.
The Telescope Control System is responsible for the coordination and control of the various subsystems that make up the telescope, responding to commands received from the Observatory Control System and from expert user interfaces.
The Telescope Control System consists of a sequencer and status/alarm monitor, a pointing kernel, a corrections module, and several adaptors. The sequencer and status/alarm monitor provide high level control of the mount, the primary, secondary and tertiary mirrors, and the enclosure (including the cap, base, shutter and vents).
The pointing kernel converts target positions (right ascension and declination) into pointing and tracking demands in the appropriate coordinate systems for the telescope mount, instrument rotators, atmospheric dispersion correctors, instrument and adaptive optics system wavefront sensor probes, and the enclosure cap and base.
The corrections module is responsible for the creation and management of the look-up tables that control the position and shape of the primary, secondary and tertiary mirrors as a function of zenith angle and temperature. It will also process data from the telescope global metrology system and provide appropriate position information to the other control systems.
The Primary Mirror Control System maintains the overall shape of the segmented primary mirror despite structural deformations caused by temperature and gravity, and disturbances from wind and vibrations (observatory-generated and seismic). It can be considered a stabilization system that works to maintain the shape of the primary mirror based on previously determined set-points, which vary as a function of zenith angle and temperature. The Alignment and Phasing System uses starlight to make measurements from which the control set-points can be determined.
Three out-of-plane motions (piston, tip and tilt) of each segment are actively controlled by the Primary Mirror Control System via three high-precision actuators per segment. Nanometer-level feedback is provided by two sensors per inter-segment edge. In total, the Primary Mirror Control System contains 1476 actuators and 2772 sensors.
The Primary Mirror Control System also controls the segment active optics warping harnesses, based on measurements made by the Alignment and Phasing System.
The Mount Control System provides servo control of the mount azimuth and elevation axes by closing a position loop around the telescope mounted motors and encoders to follow pointing and tracking commands from the Telescope Control System. The Mount Control System will do pointing and acquisition, open-loop tracking, closed-loop tracking (i.e., guiding), offsetting, and nodding motions.
Telescope motions are driven by azimuth and elevation torques supplied by linear motors mounted in the telescope structure. The elevation axis motors will be located on both the left and right elevation rockers. Position feedback will be via tape encoders mounted around the base of the telescope and along both rockers.
Because TMT has been designed as an active telescope, virtually all of its functions depend on the performance of these control systems. Accordingly, the TMT Project is giving considerable attention to this aspect of the Observatory, particularly in the system engineering area.