\begin{table}%t1
\caption{\label{tbl_log}Observation log.}
\par
%\centering
\par
\begin{tabular}{cccc}
\hline\hline \noalign{\smallskip}
Start UT date and time & $t_{\rm exp}(s)$ & Start UT date and time & $t_{\rm exp}(s)$ \\
\hline
\multicolumn{4}{c}{ESO} \\
2009 Oct. 22 04:51:00 & 1200 & 2009 Oct. 22 06:42:14 & 1200  \\
2009 Oct. 22 05:12:09 & 1200 & 2009 Oct. 23 04:50:11 & 1200  \\
2009 Oct. 22 05:35:52 & 1200 & 2009 Oct. 23 05:11:20 & 1200  \\
2009 Oct. 22 05:57:00 & 1200 & 2009 Oct. 23 05:40:44 & 1200  \\
2009 Oct. 22 06:21:05 & 1200 & 2009 Oct. 23 06:01:52 & 1200  \\
\hline
\multicolumn{4}{c}{KPNO} \\
2010 Mar. 04 03:19:37 & 1800 & 2010 Mar. 05 03:53:24 & 1800  \\
2010 Mar. 04 03:50:25 & 1800 & 2010 Mar. 05 04:58:15 & 1800  \\
2010 Mar. 04 04:25:32 & 1800 & 2010 Mar. 06 02:42:39 & 1800  \\
2010 Mar. 05 02:53:19 & 1800 &  \\
\hline
\end{tabular}
\end{table}