Closed-loop ground layer adaptive optics simulations with elongated spots : impact of modeling noise correlations

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching, GERMANY
² Centre de Recherche Astrophysique de Lyon - Observatoire de Lyon, 9 Avenue Charles André 69561, Saint Genis Laval cedex, FRANCE

^a e-mail: cbechet@eso.org

Abstract

Laser guide stars on an Extremely Large Telescope (ELT) produce elongated spots on the subapertures of the Shack-Hartmann wavefront sensors for adaptive optics (AO) correction. This degrades the measurement accuracy and introduces structured correlations in the centroiding errors, depending on the position of the laser launch telescope with respect to the sub-aperture location inside the pupil. Advanced centroiding algorithms are used to improve the measurement accuracy, but closed-loop control methods in adaptive optics usually neglect the structured error correlations. However, properly modeling the noise correlations is expected to improve the correction. We take into account structured errors statistics inside a fast algorithm for wavefront reconstruction and control on a 42-meter telescope. The closed-loop performance of the new algorithm, including a maximum a posteriori reconstruction and a pseudo open-loop control with an integrator is compared to the uniform and uncorrelated noise model used in classical AO corrections. End-to-end simulations are used to compare the two approaches. A ground layer AO configuration is simulated. The results provide additional information to the discussion about the choice of laser launch telescope positions on an ELT-class telescope.