\begin{table}%t1
\caption{\label{tab:table1}Initial optical to radio relationships$^{a}$.}
\par
\small%\centerline
 {
\begin{tabular}{l r r r}
\hline \hline
 Tracer & B1.0 to RadPos & GSC2.3 to RadPos & DR5 to RadPos \\
\hline  
N & 1962 & 2028 & 300 \\
$\overline{\Delta\alpha \cos\delta}$ & --35.2 $\pm$ 4.7 & +32.9 $\pm$ 5.5 & --4.5 $\pm$ 3.2 \\
$\overline{\Delta\delta}$ & +123.5 $\pm$ 4.9 & +39.5 $\pm$ 4.7 & +13.9 $\pm$ 3.2 \\
A1 & +4.7 $\pm$ 6.2 & --6.6 $\pm$ 6.7 & --3.5 $\pm$ 4.7 \\
A2 & +5.9 $\pm$ 5.8 & +13.1 $\pm$ 6.3 & --19.4 $\pm$ 5.1 \\
A3 & +42.1 $\pm$ 5.7 & --29.0 $\pm$ 6.1 & +6.6 $\pm$ 4.1 \\
A4N & +123.0 $\pm$ 5.3 & 20.9 $\pm$ 5.7 & 1.6 $\pm$ 4.5 \\
A4S & +128.0 $\pm$ 11.9 & 134.1 $\pm$ 12.5 & ~ \\
\hline 
\end{tabular}}
\medskip
$^{a}$ For the catalogs USNO B1.0, GSC2.3, and SDSS DR5 relatively to the collected sample of sources with precise radio interferometric positions (Extended Radio Frame~-- RadPos, either from ICRF-Ext2, VCS6 or VLAC), are presented: the average offsets in right ascension and declination, the orientation angles~(A1$-$A3) relative to the standard thriedron of equatorial coordinates, and
the equatorial biases (A4N and A4S, north and south of~$-$20$^{\circ}$). All values are in milli-arcsec (mas) and the number of sources is given in the first line.
\end{table}