Performance analysis of the segment alignment control system for the ten-meter telescope

The W. M. Keck Observatory Ten-Meter Telescope (TMT) will be a high-performance astronomical telescope capable of unprecedented ground-based observations. One of the unique features of the TMT is its primary mirror which is composed of 36 hexagonal segments. A segment alignment control system will be used to achieve the optical quality of a glass monolith in the segmented primary mirror. The problems associated with the accurate pointing and imaging of a large flexible telescope such as the TMT are quite different from those posed by traditional rigid telescopes. These problems arise from two main sources: structural dynamic effects, such as enhanced sensitivity to external disturbances, and interactions between the segment alignment control system and the telescope structure. This study consists of a combined structural and control system analysis based upon mathematical models of the TMT structure, the segment alignment control system, and the aerodynamic loads on the primary mirror that are induced by the wind. Data for the structural model were derived from the most recent design iteration available during the time frame of the study. Actual wind velocities measured at three observatory sites were used to develop a generic model of the power spectrum of forces generated by wind loads. Information about the r.m.s. mirror figure errors and limits on the bandwidth of the control system as it is currently implemented are presented.