\begin{table}%t3
\caption{\label{t3} Astrometric calibration.}
\small%\centerline
{
\begin{tabular}{lllllllll}
\hline\hline\noalign{\smallskip}
Observatory$^1$ & Astrometric    & Date & Scale          & $\sigma$(Scale) & Orient.   & $\sigma$(Orient.) &  N\_Stars & Source        \\
                & standard field &      & ($\arcsec$/px) & ($\arcsec$/px)  & ($\degr$) & ($\degr$)         &          & of coordinates \\
\hline
NAO   & M~15          &  1998 & 0.311 & 0.002 & --1.68   & 0.03   &  13        & Guide Star Catalogue (V7)   \\
NAO   & NGC~1647      &  2000 & 0.310 & 0.001 & --1.5    & 0.1    &  3$^2$     & \citet{ge}                   \\
NAO   & M~15          &  2001 & 0.310 & 0.001 & --2.04   & 0.02   &  8         & \citet{ca}                   \\
NAO   & M~67          &  2002 & 0.310 & 0.001 &  0.716  & 0.003  &  9         & Tycho Catalogue (\citealt{hc}) \\
NAO   & M~67          &  2004 & 0.314 & 0.001 &  1.42   & 0.02   &  15+$^3$   & \citet{gi}                   \\
AOB   & NGC~1647      &  1998 & 0.493 & 0.01  &  2.29   & 0.1    &  10$^4$    & \citet{ge}\\
AOB   & M~15          &  1999 & 0.495 & 0.001 &  0.4    & 0.1    &  5         & Guide Star Catalogue (V7)   \\
AOB   & M~16          &  2000 & 0.493 & 0.001 & --1.62   & 0.04   &  9         & \citet{hi}                   \\
\hline
\end{tabular}}
\smallskip
$^1$ NAO Rozhen: {2-m telescope};  -- AO Belogradchik: {0.6-m telescope};
$^2$ Field I; -- $^3$ Fields I and III;  -- $^4$ Fields I and IV.
\end{table}