\begin{table}%t5
\caption{\label{perflim}Estimation of typical limiting magnitude as a function of the different spectral resolution modes.}
\small
  %\centerline
{
  \begin{tabular}{cccc}
  \hline\hline
   Resolution & $R$ & Typical lim. magnitude & Best perf.\\
  \hline
   Low & 1700 &  6.8 & 7.5\\
   Medium & 6000 & 6.5 & 7.5\\
   High & 30~000 & 4.2 & 5.5\\
  \hline
  \end{tabular}}
  \smallskip
   These values are presented for the median value of $r_0$  at Mount Wilson i.e. 8~cm. We also indicate the best performances assuming an $r_0$ of~15~cm.  
\end{table}