\begin{table}%T1
\caption{Halfwave plate achromatization for various wavelength
ranges from the $V$ band up to the $N$ band. The table summarizes the
optimized thicknesses for the chosen doublet materials. The last
line gives the total rejection factor.\label{table1}}
%\centering
\par
\begin{tabular}{cccccc}
\hline\hline
Filters  &500--900 nm ($R=1.75$)   &700--2500 nm ($R=0.9$)  &$H$
($R=4.7$) &$K$ ($R=5.5$) &$N$ ($R=4.8$) \\\hline
Couple (a/b) &\multicolumn{4}{c}{Quartz/MgF$_2$} &CdS/CdSe
\\%\hline
$d_{\rm a}$ ($\mu $m) &$841.1$ &$1488.7$   &$1332.2$ &$1219$
&$1798.1$\\%\hline
$d_{\rm b}$ ($\mu $m) &$674.7$ &$1168$   &$1057.9$ &$977.1$
&$1618.8$\\%\hline
$\tau$ (total rejection) &$340$  &$260$ &$7900$ &$1.47\times
10^{5}$ &$2.16\times 10^{5}$
\\\hline
\end{tabular}
\end{table}