\section{Atmospheric problems}

\begin{itemize}
\item wavefront distorsions\\
Fried parameter $r_{0}$ (pathlength difference of $\lambda/2.4$
$r_{0}$ depends on the seeing and the wavelength: at $\lambda=500$~nm
$r_{0}$ is typically 10~cm (toward zenith) at average observing sites\\
\\
at $\lambda=2200$~nm, $r_{0}=50~cm$

\item coherence time\\
coherence time $t_{0}$ (the time during the atmosphere is "frozen")
$t_{0}\simeq 10$~ms at $\lambda=500$~nm
\end{itemize}



