Hybrid LQG/integrator control for the VLT eXtrem AO system SPHERE

The SPHERE (Spectro-Polarimetry High-contrast Exoplanet Research) instrument is an ESO project aiming at the direct detection of extra-solar planets. It will equip one of the four VLT 8-m telescopes in 2011. The heart of the SPHERE instrument is its eXtrem Adaptive Optics (XAO) SAXO (SPHERE AO for eXoplanet Observation) subsystem which aims at correcting any atmospheric and system defect. It should deal with a tight error budget, particularly concerning the optical axis stability. To fulfill SAXO challenging requirements a mixed control law has been designed. While a classic optimized modal gain integrator ensures the control of the deformable mirror, a linear quadratic Gaussian control has been designed to manage the tip-tilt mirror. It thus provides correction of turbulence induced optical axis fluctuations but also filtering of vibrations. A specific scheme has been developed to identify on-line the prior models for the turbulent and vibration components. We describe this original control architecture and present expectable performance of SAXO.