\begin{table}%t9
\caption{Number of nights and  hours which have
 been devoted to the study of \object{NGC~6791} as a function of the
 diameter of the telescope used for the survey.
 We adopted a mean of $5$ hours of observations per night.
\label{tab:comparison}
}
\par
%\begin{center}
\par
\begin{tabular}{c c c c c}
\hline\hline
Telescope & Diameter(m) & $N_{\rm nights}$ & Hours & Ref.\\
\hline
FLWO &   1.2     &     84     & $\sim$300 & M05\\
Loiano &   1.5     &      4     &      20 & This paper \\
SPM &   2.2     &      8     &      48 &  This paper\\
NOT &   2.54    &      7     &      35 & B03 and this paper\\
CFHT &   3.6     &      8     &      48 &  This paper \\
MMT &   6.5     &      3     &      15 & Hartmann et~al.~(2005)\\
\hline
\par
\end{tabular}
%\end{center}
\par
\end{table}