\begin{table}%T1
%\tabcolsep 0.20truecm 
\caption{Journal of observations of NGC 6996 and standard star fields together 
with calibration coefficients.}\label{tab1} 
%\centering
\par
{
\begin{tabular}{ccccccc} 
\hline \hline 
\multicolumn{1}{c}{Field}         & 
\multicolumn{1}{c}{Filter}        & 
\multicolumn{3}{c}{Exposure time} & 
\multicolumn{1}{c}{Seeing}        &
\multicolumn{1}{c}{Air-mass}       \\
 & & \multicolumn{3}{c}{[s]} & [$''$] & \\ 
\hline 
NGC 6996          & $U$ &  $900\times2$ & 180 & 10 & 2.2 & 1.091 \\
1 frame           & $B$ &  600   &  60 & 10 & 2.1 & 1.062 \\
Asiago            & $V$ &  300   &  30 &  5 & 2.2 & 1.052 \\
8/11/02           & $I$ &  300   &  30 &  5 & 2.3 & 1.045 \\
[1 ex]
NGC 6996          & $U$ &  900   &   -- &  -- & $\sim $3 & 1.368 \\
2 frames          & $B$ &  600   &   -- &  -- & $\sim $3 & 1.322 \\
Teramo            & $V$ &  300   &  60 &  -- & $\sim $3 & 1.251 \\
5-12-2002          & $I$ &  180   &  60 &  -- & $\sim $3 & 1.219 \\
[1 ex]
NGC 6996          & $U$ &  1200  & 120 &  -- & 1.9 & 1.007 \\
1 frame           & $B$ &  800   &  80 &  8 & 1.8 & 1.018 \\
Asiago            & $V$ &  600   &  60 &  6 & 1.8 & 1.027 \\
18/09/03          & $I$ &  400   &  40 &  4 & 1.7 & 1.037 \\
\hline 
PG 0231+051       & $U$ &  800   &     &    & 2.5 & 1.348 \\
Asiago            & $B$ &  300   &     &    & 2.4 & 1.324 \\ 
8/11/02           & $V$ &   60   &     &    & 2.2 & 1.316 \\ 
                  & $I$ &   90   &     &    & 2.2 & 1.315 \\ 
[1 ex]
PG 2213-006       & $U$ &  600   &     &    & 2.5 & 1.447 \\
Asiago            & $B$ &  150   &     &    & 2.3 & 1.457 \\ 
8/11/02           & $V$ &   30   &     &    & 2.3 & 1.465 \\ 
                  & $I$ &   30   &     &    & 2.3 & 1.472 \\ 
\hline
%\hline
Calibration       & \multicolumn {3}{l}{$u_1 = +3.861 \pm 0.015$}     & \multicolumn {3}{l}{$b_1 = +1.602 \pm 0.004$} \\
coefficients      & \multicolumn {3}{l}{$u_2 = -0.142 \pm 0.022$}     & \multicolumn {3}{l}{$b_2 = +0.038 \pm 0.006$} \\
Asiago            & \multicolumn {3}{l}{$u_3 = +0.58$}                & \multicolumn {3}{l}{$b_3 = +0.29$}            \\
8/11/02		& \multicolumn {3}{l}{$v_{1bv} = +1.003 \pm 0.014$} & \multicolumn {3}{l}{$i_1 = +1.691 \pm 0.044$} \\
                  & \multicolumn {3}{l}{$v_{2bv} = -0.016 \pm 0.018$} & \multicolumn {3}{l}{$i_2 = +0.057 \pm 0.043$} \\
                  & \multicolumn {3}{l}{$v_3 = +0.16$}                & \multicolumn {3}{l}{$i_3 = +0.08$}            \\
                  & \multicolumn {3}{l}{$v_{1vi} = +1.002 \pm 0.016$} & \\
                  & \multicolumn {3}{l}{$v_{2vi} = -0.013 \pm 0.016$} & \\
\hline
\end{tabular}}
\end{table}