\begin{table}%t4
\caption{Fundamental photon noise limits $\tilde{\sigma}^{\rm lim}_{\Delta\alpha/\alpha}$ 
(in units of~$10^{-5}$)  for \ion{Fe}{ii}~pairs of lines ($\lambda1608/X$) from the \zabs~= 1.839 system toward \object{Q~1101--264}. The exposure time~$t_{\rm exp}$ and the total efficiency~$\varepsilon_{\rm tot}$ are set to 3600~s and~0.15, respectively.}\label{tbl-4}
%\centering
$$
\begin{tabular}{lr@{.}l r@{.}l r@{.}l r@{.}l}
\hline
\hline
\multicolumn{1}{r}{Line:}  & \multicolumn{2}{c}{$\lambda1608$} & 
\multicolumn{2}{c}{$\lambda2382$} & 
\multicolumn{2}{c}{$\lambda2586$} & 
\multicolumn{2}{c}{$\lambda2600$} \\
\hline
$W_{\rm obs}$, \AA & 0&0446 & 0&2779 & 0&1239 & 0&2770 \\
$\Delta\lambda_{\rm pix}$, \AA & 0&05 & 0&06 & 0&05 & 0&05 \\
$\xi$ & 1&83 & 1&83 & 1&93 & 1&93 \\
$Q$-factor & 0&11 & 0&58 & 0&29 & 0&64 \\
$\sigma^{\rm lim}_\lambda$, m\AA & 5&7 & 1&3 & 2&3 & 1&0 \\
$\tilde{\sigma}^{\rm lim}_{\Delta\alpha/\alpha}$ & \multicolumn{2}{l}{--} &
1&1 & 1&0 & 1&1 \\
\hline
\end{tabular}
$$
\end{table}