\begin{table}%t2
%\centering
\par
\caption{\label{tab:spectral_index}HST-1 $\lambda$20$-$$\lambda$2~cm spectral index\tablefootmark{a}.}
\begin{tabular}{ccccc}
\hline\hline
\noalign{\smallskip}
\multicolumn{2}{c}{Epoch}        &  \multicolumn{2}{c}{$S_{{\rm \nu}}$ \footnotesize{[mJy]}} & $\alpha$  \\
$\lambda$2~cm & $\lambda$20~cm &  $\lambda$2~cm & $\lambda$20~cm &           \\
\hline\\[-3.25mm]
2005.30        & 2005.35         &  $19.7 \pm 1.0$             &   $111 \pm 6$\tablefootmark{b}  &  $-0.75 \pm 0.03$ \\
2005.85    & 2005.82         &  $19.9 \pm 1.0$             &   $126 \pm 6$\tablefootmark{b}  &  $-0.80 \pm 0.02$ \\
\hline
\end{tabular}
\tablefoot{
\tablefoottext{a}{The spectral index and its errors are formal, see Sect.~\ref{sec:spectra};}
\tablefoottext{b}{5\% error assumed for VLBI observations.}}
\end{table}