\begin{table}%t2
  %\centering
\par
  \caption{Number of photons per frame and per channel needed for the
    frame to contribute to the ensemble averages, as a function of the
    observing mode and considering the measured and the potential
    (vibration-free) atmosphere+VLTI+AMBER visibility.}\label{tab:needed}
  \begin{tabular}{lcc}
    \hline \hline
    Mode & actual Visibility& potential Visibility\\
  \hline
  Low   & $V_{\rm actual}=0.12$ & $V_{\rm potential}=0.5$\\
  Resolution   & 581~phots  & 187 phots\\
  \hline
  Medium   & $V_{\rm actual}=0.14$ & $V_{\rm potential}=0.6$\\
  Resolution   & 507 phots  & 166 phots\\
  \hline
  \end{tabular}
\end{table}