\begin{table}%t6
  %\centering
\par
  \caption {\label{meanz}Mean and median redshifts in the deep and wide fields,
    for several limiting magnitudes. Both values are computed directly 
    from the data.}
  \begin{tabular}{c c c c c }
   & \multicolumn{2}{c}{deep} & \multicolumn{2}{c}{wide}\\
     \hline
      \hline
      $i'_{\rm AB} < $ & $\langle z\rangle $ & $z_{\rm median}$ & $\langle z\rangle$ & $z_{\rm median}$\\
      \hline
      20.5 & 0.36  & 0.34  & 0.37 & 0.35 \\
      21.0 & 0.40 & 0.37 & 0.41 & 0.39 \\ 
      21.5 &  0.46 & 0.42 & 0.46 & 0.44 \\
      22.0 & 0.51 & 0.47 & 0.51 & 0.49 \\
      22.5 & 0.58 & 0.54 & 0.56 & 0.55 \\
      23.0 & 0.65 & 0.62 & 0.62 & 0.60 \\
      23.5 &  0.74 & 0.70 & 0.68 & 0.65 \\
      24.0 &  0.84 & 0.77 & 0.73 & 0.70 \\
      24.5 & 0.93 & 0.83 & 0.77 & 0.75 \\ 
      25.0 & 1.00 & 0.88 & 0.81 & 0.70 \\
      \hline
   \end{tabular}
\end{table}