\begin{table}%t3
\caption{Characteristics of the co-added WFI images. The limiting
magnitudes quoted in Col.~4 are defined in the Vega system via
$m_{\rm lim}=ZP-2.5\log(\sqrt{N_{\rm pix}}\cdot 3\cdot\sigma)$,
where $ZP$ is the magnitude zeropoint, $N_{\rm pix}$ is the number
of image pixels in a circle with radius $2\myarcsec 0$ and $\sigma$
the sky background noise. The seeing in Col.~5 was measured with
the {\tt SExtractor} FWHM\_IMAGE parameter. The groups in Bonn (B)
and Innsbruck~(I) observed Z3146 with an offset of about $5\myarcmin
0$ In Ra and $8\myarcmin 0$ in Dec. The exact layout is given in
Fig.~\ref{fig:layout}. For the $R$ band we created, besides the
individual stacks, a deep mosaic in the common area (A). We note
that the Innsbruck $R$~band has nearly double the exposure time of
the Bonn image but about the same limiting magnitude as it was
observed during less favourable moon phases. The quality of our
photometric calibration is crosschecked in Fig.~\ref{fig:photometry}.\label{tab:data}}
\small%\centering
\par
\begin{tabular}{ccccc}
\hline \hline
Filter & Image region & exp. time (s) & limiting magnitude ($3\sigma$ in a $2\myarcsec 0$ aperture) & Seeing (arcsec)\\
\hline
$B$ & (B) & 1500 & 24.70 & 1.21 \\
$V$ & (I) & 8040 & 25.02 & 1.16 \\
$R$ & (I) & 16077 & 25.27 & 1.02 \\
$R$ & (B) & 8850 & 25.24 & 1.11 \\
$R$ & (A) & 24927 & 25.71 & 1.04 \\
\hline
\end{tabular}
\end{table}