\begin{table}%t3
\caption{Observation log.}\label{tab:OB} 
\small
%\centerline
 {\begin{tabular}{cccc}
\hline \hline
Cluster&  Setup  &  Exposure & Run dates  \\
&  &  time$^{a}$  (m) &  \\
\hline
$\sigma$ Ori &  HR15N &43.2 &2004--10--01 \\
 $\sigma$ Ori & HR15N & 40.3 &   2004--12--01 \\
 $\sigma$ Ori & HR15N & 43.2 &   2004--12--02 \\
 $\sigma$ Ori &  HR15N & 43.2 &   2004--12--03 \\
 $\sigma$ Ori & HR15N & 43.2 &   2004--12--04 \\
 $\sigma$ Ori & HR15N & 43.2 &   2004--12--09 \\
\hline
$\lambda$ Ori & HR15N & 46.0 &2005--10--15 \\
$\lambda$ Ori & HR15N & 46.0  &  2005--10--16 \\
$\lambda$ Ori & HR15N &46.0  &  2005--10--17 \\
$\lambda$ Ori &  HR15N & 46.0  &  2005--11--10 \\
$\lambda$ Ori &  HR21 &46.0 &2005--11--12 \\
$\lambda$ Ori & HR21 &46.0  &  2005--11--12 \\
$\lambda$ Ori & HR21 & 46.0  &  2005--11--12 \\
$\lambda$ Ori &  HR21 & 46.0  &  2005--11--23 \\
\hline
\end{tabular}}
\medskip
$^a$ Excluding instrument overheads.
\end{table}