\begin{table}%t9
\caption{\label{cat_sigma}Determination of the astrometric accuracy of the catalogues used in the LQAC.} 
\small%\centerline
 {      
\begin{tabular}{lrrrrrr}
\hline
\hline
& \multicolumn{2}{c} {Mean (mas)} & \multicolumn{2}{c} {$\sigma$ (mas)} & & \\
& $\Delta\alpha\cos{\delta}$ & $\Delta\delta$ & $\Delta\alpha\cos{\delta}$ & $\Delta\delta$ & $N_1$ & $N_2$ \\	 
\hline
VLBA  &$ -0.010 $&$ 0.039 $&$ 0.767 $&$ 0.585 $&$       94 $&$ 4 $\\
VLA   &$ -0.054 $&$ -0.009 $&$ 1.025 $&$ 1.391 $&$      63 $&$ 10 $\\
JVAS   &$ -3.287 $&$ -0.081 $&$ 9.793 $&$ 12.629 $&$     90 $&$ 5 $\\
SDSS  &$ 1.210 $&$ -12.203 $&$ 52.022 $&$ 51.728 $&$    96 $&$ 4 $\\
2QZ   &$ 86.242 $&$ 45.991 $&$ 193.667 $&$ 181.214 $&$  98 $&$ 3 $\\
FIRST &$ -30.282 $&$ 0.010 $&$ 286.750 $&$ 319.342 $&$  96 $&$ 3 $\\
HB    &$ 97.800 $&$ 100.152 $&$ 726.512 $&$ 785.789 $&$ 85 $&$ 5 $\\
VV06  &$ 30.393	$&$ 286.513 $&$ 582.571 $&$ 586.322 $&$ 97 $&$ 3 $\\
\hline
\end{tabular}}
\medskip
$N_1$ stands for the percentage of quasars remaining after a $3\sigma$~rejection threshold algorithm, and $N_2$ for the number of necessary iterations.
\end{table}