Active Optics of Large Segmented Mirrors: Dynamics and Control

This paper examines the active optics of future ELT segmented mirrors from the point of view of dynamics simulation and control. A reduced order structural model, fully repre-sentative of the quasi-static behavior and the low frequency dynamics, is obtained by Craig-Bampton reduction. Assuming an array of edge-sensors and an array of normal sensors, two control options are discussed: (1) A dual loop controller combining a SVD controller of the Jacobian of the edge sensor, and a Zernike modes controller based on the normal sensors, con-trolling the low order aberrations which are unobservable from the edge sensors. (2) A SVD controller of the extended Jacobian combining the edge sensors and the normal sensors. Both of these controllers are based on the quasi-static behavior of the mirror. The control-structure interaction is addressed through a frequency domain robustness test for MIMO systems; this test separates the dynamic response (residual) from the quasi-static response (primary) of the mirror. This allows to clearly identify the flexible modes which are critical for stability and to study the influence of structural changes in the supporting structure (stiffness and damping). This discussion is intended to be generic rather than targeted to a specific telescope.