\begin{table}%t6
\caption{\label{tab:fit}Metallicity calibrations.}
%\centering
\small
\begin{tabular}{l l @{~~= ~~} r@{ } r@{ } r@{ }}
\hline\hline
Type& \multicolumn{4}{c}{Calibration}\\
\hline
%&&&& \\[-8pt]
quadratic&  \feh$^{\rm  ZW}$&$0.088 ~{W^\prime}^2$&$-\;0.184 ~ W^\prime$&$-\;2.079$\\
&\feh$^{\rm  CG}$&$0.072 ~{W^\prime}^2$&$-\;0.076 ~ W^\prime$&$-\;2.122$\\
&\mh             &$0.051 ~{W^\prime}^2$&$+\;0.056 ~ W^\prime$&$-\;2.125$\\
linear&
 \feh$^{\rm  ZW}$  &&$0.359~ W^\prime$&$-\;2.845$\\
&\feh$^{\rm  CG}$  &&$0.391~ W^\prime$&$-\;2.806$\\
&\mh               &&$0.395~ W^\prime$&$-\;2.628$\\
\hline
\end{tabular}
\end{table}