\begin{table}%t1
\caption{Observation log.}\label{Tab1}
\small%\centerline
{
\begin{tabular}{lcccc}
\hline \hline 
Instrument &  Exposure [ks]& Net exposure [ks]$^a$ \\
\hline
PN &  62   & 45  \\
MOS1 & 67  & 49     \\
MOS2  & 67   & 51     \\
\hline
\end{tabular}}
\medskip
$^a$ Net on-axis exposure time after rejection of high background periods (see
Sect.~2).  
\end{table}