Chromatic anisoplanatism in Adaptive Optics

Chromatic effects can seriously limit Adaptive Optics performance when a telescope is used at large zenith angles, either because of the wavelength range being corrected or because the laser guide star (LGS) is at a different wavelength to the science observation. There are two types of Refractive effect - the first is due to the fact that the refractive index of air depends on wavelength, giving rise to different wavelengths having not having the same path length. The second, referred to here as chromatic anisoplanatism, is due to the fact that bending of light due to atmospheric refraction causes rays of different wavelengths to take different paths through the turbulence. In this article we consider chromatic anisoplanatism in more detail, and extend the analysis to consider a spherical rather than a planar atmosphere model. As a particular case we investigate the effect for the ESO Extremely Large Telescope, employing a layered model for the turbulence above the ELT site at Cerro Armazones.