\begin{table}%Ta2
\caption{\label{gainfactor}Periods of gain stability and correction factors ($=F_0/F_{\rm HI}$).}
\par
%\centering
\par
\begin{tabular}{cc}
\hline\hline
before June 30th 1991 & 0.943 \\
 Jul. 1991--Oct. 1992 & 1.000 \\
 Nov. 1992--Nov. 1994 & 1.143 \\
 Dec. 1994--Nov. 1995 & 1.290 \\
 Dec. 1995--Nov. 1996 & 1.449 \\
 Dec. 1996--Jun. 1999 & 1.508 \\
\hline
\end{tabular}
\end{table}