\begin{table}%t2
\caption{Phase calibrators, beam sizes, rms noise, and number of radio sources detected.   
\label{tab:cores}      }
%\centerline
 {
\begin{tabular}{l c ccc cccc c c ccc}
\hline\hline        
\noalign{\smallskip}
Core &
\multicolumn{3}{c}{Phase calibrator(s)}  &&  
\multicolumn{2}{c}{Beam (\arcsec)}&&
\multicolumn{2}{c}{rms noise ($\mu{\rm Jy/bm}$)$^a$
}&
 &\multicolumn{2}{c}{Radio sources$^b$}\\    
\noalign{\smallskip}
\cline{2-4}\cline{6-7}\cline{9-10}\cline{12-13}
\noalign{\smallskip}
& Calibrator &$S_{\rm 3.6\ cm}$(${\rm Jy}$) &$S_{\rm 6\ cm}$ (${\rm Jy}$)& 
&3.6 cm &6 cm & & 3.6 cm & 6 cm  && 3.6 cm & 6 cm  \\
\noalign{\smallskip}\hline                        
\noalign{\smallskip}
L1582A &
$0530+135$& $2.905\pm0.002$ & $3.08\pm0.09$&&
$3.3\times2.7$&$12.4\times6.6$ &&13 & 29 &&4 (2.1)&2 (2.7) \\
\noalign{\smallskip}
L1689A &
$1522{-}275$&$1.210\pm 0.006$ &$1.06\pm0.07$  &&
$5.5\times2.9$&$17.1\times 10.4$ &&12 & 60&&6 (2.3) &5 (1.3) \\
&$1626{-}298$&$2.602\pm0.015$&--&&&&&&\\
\noalign{\smallskip}
B133   &
$1939{-}100$ & $0.566\pm0.002$ &-- && 
$3.7\times3.0$&$16.9\times9.6$ &&11 & 60 &&4 (2.3) &2 (1.3)\\
&$1939{-}154$&--& $0.55\pm0.01$&&&\\
\noalign{\smallskip}
B68    &
$1743{-}309$ &$0.233\pm0.001$ &--&& 
$5.3\times3.0$& -- &&12 & --&&4 (2.3)&-- \\
&$1751{-}253$& $0.259\pm0.001$ &--&&&\\
\hline                        
\end{tabular}}
\par
\smallskip
$^a$ {Mean rms noise in the field in $\mu{\rm Jy/beam}$};
$^b$ {number of radio sources detected in each field. 
In the parentheses
is the expected number of background sources calculated using 
Eqs.~(\ref{eq:back1}),~(\ref{eq:back2}).}
\end{table}