\begin{table}%t1
\caption{\label{data}SUMER observations of temporal series in polar coronal 
holes (PCHs).}
%\centering
\par
\begin{tabular}{ccccccc}
\hline\hline
  Item & Date & Time & Solar coordinate & Wavelength used  &  Detector & Exposure \\
   &  & UT & $x$ (\arcsec), $y$ (\arcsec) & ({\AA}) & and slit & time (s) \\
\hline
PCH~1 & 20/10/1996 & 19:57--23:57 & 0, 909.75 N & \ion{N}{iv}~765~\AA, 
\ion{Ne}{viii}~770~\AA\ & B 2 (1\arcsec~$\times$~300\arcsec) & 30\\
PCH~2a & 25/02/1997 & 00:03--13:58 & 0, 950.25 S & \ion{N}{iv}~765~\AA  
& B 2 (1\arcsec~$\times$~300\arcsec) & 60 \\
PCH~2b & 21/02/1997 & 01:20--01:35 & 0, 984.75 S & \ion{C}{iii}~977~\AA\  
& B 6 (0.3\arcsec~$\times$~120\arcsec) & 15 \\
\hline
\end{tabular}
\end{table}