\begin{table}%t1
\caption{\label{tab:obs}Mrk~509 observations.}
%\centering
\par
\begin{tabular}{l c c c c c c}  
\hline\hline   
\noalign{\smallskip}                 
Observation & Date & Obsid & Duration & Exposure RGS\tablefootmark{a} & Exposure pn\tablefootmark{a} & Flux\tablefootmark{b}  \\    % table heading 
                       &	        &	     & (ks) & (ks) & (ks) & (W m$^{-2}$ s$^{-1}$) \\
\hline\\[-3.2mm]                       % inserts single horizontal line
   1 & 18-10-2005 & 0306090201 & 86 & 85 & 58 & $7.1 \times 10^{-14}$  \\      % inserting body of the table
   2 & 20-10-2005 & 0306090301 & 47 & 47 & 32 & $7.2 \times 10^{-14}$  \\
   3 & 24-04-2006 & 0306090401 & 70 & 67 & 43 & $8.1 \times 10^{-14}$  \\
\hline 
   \end{tabular}                               %inserts single line
\tablefoot{\tablefoottext{a}{Effective exposure time after filtering.} 
\tablefoottext{b}{The flux is given for the pn 0.2--10 keV band.}}
\vspace*{-2mm}
\end{table}