\begin{table}%t1
%\centering
\par
  \caption{Summary of the Keck astrometry. }
  \label{tab:summary}
  \begin{tabular}{lccc}
    \hline
    \hline
    Object & $x$(\arcsec) & $y$(\arcsec) &  Flux \\
    \hline
    QSO    &   $+$0.000         &    $+$0.000        &    $-$ \\
    A          &   $+$0.731         &    $+$0.027         &    1.00   \\ 
    B          &  $-$0.539        &     $+$0.132        &    0.72   \\     
    G1       &  $-$8.318         &    $+$2.873         & $-$ \\
    G2       &  $-$2.842         &    $+$1.610         & $-$ \\
    \hline
  \end{tabular}
  \tablefoot{The error bars in the positions are about 20 mas, i.e., about half a pixel.}
\end{table}