Optimisation of the control laws for the SPHERE XAO system

The SPHERE (Spectro-Polarimetry High-contrast Exoplanet Research) instrument is an ESO project aiming at the direct detection of extra-solar planets. It should equip one of the four VLT 8-m telescopes in 2010. The heart of the SPHERE instrument is its eXtrem Adaptive Optics (XAO) SAXO (SPHERE AO for eXoplanet Observation) subsystem that should deal with a tight error budget. To fulfil SAXO challenging requirements a mixed control law has been designed. It includes both an optimized modal gain integrator to control the Deformable Mirror (DM) and a Linear Quadratic Gaussian (LQG) control law to manage the tip-tilt (TT) mirror and filter possible vibrations. A specific scheme has been developed to optimize the correction provided by the DM and the TT while minimizing the coupling between both control loops. Actuator saturation and wind-up effects management are described. We describe the overall control architecture and focus on these main issues. We present expectable performance and also consider the interactions of the main control loop with other subsystems. PUBLISHER'S NOTE Sept. 9, 2010: Due to a production error, SPIE Paper 70151U was inadvertently published also as SPIE Paper 70151D. This has been corrected. This record contains the correct citation, abstract, and manuscript for paper 70151D.

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