\begin{table}%t1
%\centering
\par
\caption{\label{tab1}Log of the observations.}
%\centering
\par
\begin{tabular}{cccc} 
\hline\hline
 Date UT & Filter & Exp. time (s) & Air mass\\
\hline      
 1999 Jun. 1 06:46:24 & $V$  & 180  & 1.232 \\ 
 1999 Jun. 1 06:51:08 & $V$  & 180  & 1.246 \\ 
 1999 Jun. 1 06:55:52 & $V$  & 180  & 1.261 \\ 
 1999 Jun. 1 07:00:36 & $V$  & 180  & 1.277 \\ 
 1999 Jun. 16 02:27:00 & $R$  & 180  & 1.163 \\
 1999 Jun. 16 02:31:45 & $R$  & 180  & 1.154 \\
 1999 Jun. 16 02:36:29 & $R$  & 180  & 1.146 \\ 
 1999 Jun. 16 02:41:13 & $R$  & 180  & 1.138 \\	   
 1999 Jun. 16 02:46:07 & $V$  & 180  & 1.131 \\ 
 1999 Jun. 16 02:50:51 & $V$  & 180  & 1.124 \\ 
 1999 Jun. 16 02:55:35 & $V$  & 180  & 1.118 \\ 
 1999 Jun. 16 03:00:19 & $V$  & 180  & 1.112 \\ 
 1999 Jun. 16 03:05:13 & $R$  & 180  & 1.107 \\
 1999 Jun. 16 03:09:57 & $R$  & 180  & 1.103 \\
 1999 Jun. 16 03:14:41 & $R$  & 180  & 1.099 \\ 
 1999 Jun. 16 03:19:25 & $R$  & 180  & 1.095 \\
 1999 Jun. 16 04:06:32 & $V$  & 180  & 1.086 \\ 
 1999 Jun. 16 04:11:17 & $V$  & 180  & 1.087 \\ 
 1999 Jun. 16 04:16:01 & $V$  & 180  & 1.089 \\ 
 1999 Jun. 16 04:20:45 & $V$  & 180  & 1.092 \\ \hline
\end{tabular}
\end{table}