\begin{table}%t1
\caption {Observing log of IRS~3. The wavelength
  dependent~${\it TF}$ has been estimated for each night on
  the basis of regularly conducted calibrator measurements, typically
  about once per hour (Sect.~\ref{sec:2}). The number of calibrators used per night is
  given, and the applied parameters are given in Table~\ref{tab:2}. 
\label{tab:1}}
%\centering
                          % used for centering table
\begin{tabular}{c c c c c}        % centered columns (4 columns)
\hline\hline                 
 Baseline & PB & PA & Airmass & Accuracy\\    % table heading 
  & [m]& [deg E of N] & [1] & $[$\%$]$\\
\hline
\multicolumn{4}{l}{\#1 night: 2004-07-07 \quad -- \quad 5 calibrators} \\
U2-U3 & 45.7 & 46 & 1.03 & 12\\
U2-U3 & 42.3 & 52 & 1.16 & 12\\
U2-U3 & 37.0 & 55 & 1.45 & 12\\
U2-U3 & 26.4 & 53 & 2.63 & 14\\
\hline
\multicolumn{4}{l}{\#2 night: 2004-07-08\quad -- \quad 8 calibrators} \\
U2-U3 & 44.9 & 11 & 1.38 & 19\\
U2-U3 & 43.5 & 50 & 1.11 & 14 \\
\hline
\multicolumn{4}{l}{\#3 night: 2005-05-25\quad -- \quad 7 calibrators} \\
U3-U4 & 45.8 & 89 & 1.36 & 14\\
\hline                                   %inserts single line
\multicolumn{4}{l}{\#4 night: 2005-06-23\quad -- \quad 4 calibrators} \\
U3-U4 &  45 & 89 & 1.39 & 23\\
U3-U4 &  46.8 & 90 & 1.33 & 16\\
\hline                                   %inserts single line
\multicolumn{4}{l}{\#5 night: 2005-06-27\quad -- \quad 4 calibrators} \\
U3-U4 & 54& 139 & 1.29 & 11\\
\hline                                   %inserts single line
\multicolumn{4}{l}{\#6 night: 2005-07-20\quad -- \quad 7 calibrators} \\
U3-U4 & 58.6 & 101 & 1.07 & 22\\
U3-U4 & 59.6 & 124 & 1.08 & 22\\
\hline                                   %inserts single line
\multicolumn{4}{l}{\#7 night: 2005-08-23\quad -- \quad 2 calibrators} \\
U3-U4 &  62.5& 112 & 1.00 & 20 \\
U3-U4 &  54.9& 136 & 1.25 & 21 \\
\hline                                   %inserts single line
\end{tabular}
\end{table}