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The impact of Atmospheric dispersion on Adaptive Optics for Extremely Large Telescopes

Abstract 121

Submitted by Laurent JOLISSAINT


L. Jolissaint (1), R. Stuik (1), S. Kendrew (1), S. Hippler (2), B. Brandl (1)


(1) Leiden Observatory, The Netherlands; (2) Max-Planck-Institut fur Astronomie, Heidelberg, Germany


Dispersion of the air refractivity index has never been considered an important issue in adaptive optics, and it is generally considered that optical turbulence is achromatic. For moderate Strehl AO systems, on moderate size apertures, this is indeed true, and wavefront sensing can be done at another wavelength than the correction wavelength. But when considering high Strehls or extremely large telescope apertures, dispersion effects, which are scaling with the diameter of the telescope, become noticeable and might prevent AO systems to reach their peak performance. These effects range from ’standard’ refractivity variations at short wavelengths and near absorption bands to optical turbulence due to atmospheric composition fluctuations, mainly due to water vapor.

In this study, we will review the different effects that needs to be taken into account when refractivity cannot be considered constant anymore, quantify the impact of these effects for moderate to large and extremely large aperture and propose solutions to minimize these effects. As a test case, we will discuss the impact of refractivity effects for several instruments proposed for the E-ELT.