\begin{table}%t1
%\centering
\par
\caption{Observing parameters.\label{tab1}}
\begin{tabular}{l c}
\hline
\hline
UT date &   20 Apr. 2002 \\
\hline
$r$ (AU) & 0.89\\
$\dot r$ ($\rm km ~ s^{-1})$ & 29.06 \\
$\bigtriangleup$ (AU) &    0.43\\
$\dot \bigtriangleup$  ($\rm km ~s^{-1}$) & --8.3 \\
Exposure time (s) & 2400 \\
\hline
\end{tabular}
\end{table}