Pyramid wavefront sensing in the presence of thick spiders
One of the challenges faced by the adaptive optics systems on extremely large telescopes, such as the European Extremely Large Telescope (EELT), is the pupil segmentation caused by the secondary mirror support structure (spider). We introduce a sensitivity metric to evaluate the sensitivity of the pyramid wavefront sensor to segment piston and propose a hybrid control method where both modulated and unmodulated pyramid wavefront sensor measurements are used. The modulated measurements correct the bulk of the atmospheric turbulence whilst the unmodulated measurements are used to correct the residual segment piston errors. Our hybrid approach provides a 5.6% improvement in Strehl under poor seeing (r0 =10 cm ) and 2.7% improvement under good seeing conditions (r0 =20 cm).