Dynamic aspects of segmented mirror position control

Extreme large optical telescopes will operate in an open environment and may be excited by wind effects. The position control of the mirror segments may need fast control, and the position actuators and the related control loops may be separated in a conventional slow, iso-static and a fast reaction-mass type system. There exists some experience with wind excitations of airborne telescopes, e.g. the Stratospheric Observatory for Infrared Astronomy SOFIA. The pointing control system of that telescope is equipped with several dedicated design features, as a vibration isolations system, a flexible body control system and an active mass damper system to handle excitations in different frequency ranges. These features may be transferred to the position control systems of segmented mirrors. The paper will give some system engi-neers recommendations for designing those systems.