\begin{table}%t2
%\centering
\par
\caption{Zero points, color terms and airmasses for the
different pointings and filters. Date and airmass (\emph{X}) refer to the image taken as reference when
the relative photometry was computed. The last column gives the values to be
added to the Vega-system magnitudes to obtain \mbox{AB magnitudes.}}
\label{Tab:cal}
\begin{tabular}{ccccccccc}
\hline\hline 
Pointing&
 Date&
 Filter&
 $\alpha$&
 $\beta$&
$\varepsilon$&
 rms &
\emph{X}&
 AB corr\\
\hline
1 &
 Nov. 1999 &
 Loiano &
 0.21 $\pm$ 0.02 &
 22.15 $\pm$ 0.06 &
0.43 $\pm$ 0.02&
 0.06&
1.125&
 0.73\\
2 &
 Nov. 2000 &
 ESO U/360 &
 0.08 $\pm$ 0.02 &
 22.01 $\pm$ 0.02 &
0.49&
 0.06&
1.105&
 0.94\\
3 &
 Oct. 2001 &
 ESO U/360&
 0.08 $\pm$ 0.02&
 22.10 $\pm$ 0.02&
0.44 $\pm$ 0.03&
 0.04&
1.139&
 0.94 \\
\hline
\end{tabular} 
\end{table}