Abstract

Seeing monitoring in astronomy is widely based on the statistical analysis of angle-of-arrival (AA) fluctuations, which are usually modelled in the framework of the near-field approximation where diffraction through turbulence is ignored. They are consequently believed to be completely independent of wavelength. We discuss in this paper the influence of Fresnel diffraction from distant turbulence layers on multi-wavelength (polychromatic) AA fluctuations. For this purpose we propose a model for polychromatic AA fluctuations in weak turbulence conditions and derive an analytical model for their variance in the case where scintillation is ignored. We also present a numerical simulation that includes scintillation and justifies that this latter may be neglected in the analytical model.