\begin{table}%t2
%\vspace{.3cm}
%\centering
\par
\caption{Classification of the designated asteroids
according to the amount and timing of the available astrometry.}
\label{tab:qualdef}
\begin{tabular}[c]{cccc}
\hline
\hline
\multicolumn{1}{l}{Quality code}  & 
\multicolumn{1}{r}{No. nights}  &
\multicolumn{1}{r}{Arc length (d)}       &
\multicolumn{1}{r}{No. obs.}
\\ 
\hline  
1   & $\ge $5 & $\ge $180 & $\ge $10 \\
2   & $\ge $4 & $\ge $20  & $\ge $8  \\
3   & $\ge $3 & $\ge $6.5 & $\ge $6  \\
4   & $\ge $3 & $\ge $1.5 & $\ge $6  \\
5   & $\ge $2 & $\ge $6.5 & $\ge $4  \\
6   & $\ge $2 &	      & $\ge $3  \\
7   & $=   $1 &	      & $\ge $3  \\
\hline
\end{tabular}
\end{table}