\begin{table}%t3
\caption{Performed duration and effective exposure time of both observations 
(April and September 2002). The difference between the performed and effective 
duration of the exposures is only due to~\mbox{overheads}. \label{expt}}
%\centering
\par
\begin{tabular}{ccccc}
\hline \hline
	& \multicolumn{2}{c}{Obs.~1} & \multicolumn{2}{c}{Obs.~2} \\
\cline{2-3}\cline{4-5}
Instrument	& Performed & Effective & Performed & Effective \\
	& duration & exposure & duration & exposure \\
	& (s) & (s) & (s) & (s) \\
\hline
MOS1 & 13~122 & 12~855 & 13~667 & 13~402 \\
MOS2 & 13~122 & 12~896 & 13~667 & 13~406 \\
pn & 10~546 & 9525 & 12~046 & 10~771 \\
\hline
\end{tabular}
\end{table}