A Crossover Network based Control Concept for the Tip-Tilt Rejection in the Mid-Infrared ELT Imager and Spectrograph (METIS)

The telescope instrument Mid-infrared ELT Imager and Spectrograph (METIS) is currently under development and will enable high-resolution spectroscopic and coronagraphic imaging. To obtain diffraction-limited measurements despite wavefront disturbances, METIS uses a single conjugate adaptive optics (SCAO) system comprising the active mirrors M4/M5, a pyramid wavefront sensor, and a controller. Because tip-tilt wavefront disturbances are dominant, the main task of the METIS-SCAO system is their rejection. Since M4 and M5 can redundantly correct tip-tilt errors and M4 features much faster dynamics and smaller actuator strokes than M5, the METIS-SCAO system is a dual-stage system. In order to achieve a proper tip-tilt correction for METIS while splitting this correction efficiently between the active mirrors, we present a new crossover network (CN) based control concept. This concept assigns low-frequency high amplitude tip-tilt error components to M5 and high-frequency small amplitude components to M4 via a specific type of CNs. Subsequently, two independent single-input single-output controller command the active mirrors, such that the assigned error components are corrected. Simulations show that the presented control concept rejects tip-tilt wavefront errors well for medium observation conditions.