Main axes control of E-ELT

Associated to tracking capabilities, the main axes control system of the E-ELT is the first correcting system in the chain of control loops for reducing the image motion (tip/tilt) caused by perturbations on the telescope. The main objective of the closed-loop performance analysis of the axes is to evaluate the trade offs for the choice of control system hardware, i.e. specification and location of the motors and sensors (encoders/tachometers). In addition, it defines the design constraints and requirements (actuator stroke and bandwidth) of other correcting systems in the chain: the field stabilization (M5 unit) and adaptive deformable mirror (M4 unit). In this paper the main axes control analysis of E-ELT is presented and the performance of telescope in face of external perturbations such as wind and imperfections of the drive (cogging/ripple) and sensing (noise) systems is evaluated. The performance metric is the wavefront error at the focal plane which is derived from the mechanical motion of the telescope's optical elements together with their respective optical sensitivities.