\begin{table}%t1
\caption{\label{table1}XRT and TRACE exposure times.}      
%%\centering

\par
\begin{tabular}{c c c c c c c }      
\hline\hline\noalign{\smallskip}
&  \multicolumn{5}{c}{XRT} &  TRACE \\                        
Time (UT)& $Al\_poly$ &$C\_poly$ & $Be\_thin$ & $Be\_med$ & $Al\_med$ & 171 \\ 
\hline                    
17:55 & 0.18 & 0.4 & 1.4 & 5.8 &11.57 &  \\
10:36 & 0.26 & 0.36& 1.44& 8.19 & 16.39 &\\
10:46 &  0.18 & 0.26 & 1.03 & 5.80 & 11.57 & \\
11:01 & 0.01 & 0.02 & 0.07 & 0.36 & 0.76 & \\ 
11:09 &      &     &     &     &      & 55.1\\
11:16 &  0.03 &0.05 & 0.18 & 1.03 & 2.05 & 32.8 \\
11:21 &  0.1 &  0.13 &  0.51 &  2.90 & 5.80 &  $\vdots$ \\
11:26 & 0.36 & 0.51 & 0.36 & 2.05 & 4.10 & $\vdots$\\
11:31  &0.26 & 0.36& 1.44& 8.19 & 16.39 & $\vdots$\\
11:46 & 0.18 & 0.51 & 1.03 & 5.80 & 11.57 & $\vdots$\\
11:56 & 0.18 & 0.18 & 0.73 & 4.10 & 8.19  & $\vdots$\\
13:46 & 0.3 & 0.4 & 1.4 & 8.2 & 16.4 & $\vdots$\\
\hline                  
\end{tabular}
\tablefoot{Start time of the observation (first column), the exposure times in seconds (second-sixth columns) for XRT and TRACE (last column).}    
\end{table}