We have obtained spectra with resolution \sim 2.5 Å in the region \approx 7500–9500 Å for 116 red giants in 5 Galactic globular clusters and 6 old open clusters ( 5 with published metallicities , and 1 previously unmeasured ) .
The signal-to-noise ranges from 20 \leq S/N \leq 85 .
We measure the equivalent widths of the infrared Ca II triplet absorption lines in each star and compare to cluster metallicities taken from the literature .
With globular cluster abundances on the Carretta & Gratton scale , and open cluster abundances taken from the compilation of Friel and collaborators , we find a linear relation between [ Fe/H ] and Ca II line strength spanning the range - 2 \la [ Fe/H ] \la - 0.2 and 2.5 \la ( age/Gyr ) \la 13 .
The reference abundance scales appear to be consistent with each other at the \sim 0.1 dex level .
Alternate choices for metallicity scales can introduce curvature into the relation between [ Fe/H ] and Ca II equivalent width .
No evidence for an age effect on the metallicity calibration is observed .
Using this calibration , we find the metallicity of the massive , old , open cluster Trumpler 5 to be [ Fe/H ] = - 0.56 \pm 0.11 .
This is the first spectroscopic abundance measurement for Trumpler 5 , and is lower by \approx 0.3 dex than estimates based on the cluster colour-magnitude diagram .
Considering the 10 clusters of known metallicity shifted to a common distance and reddening , we find that the additional error introduced by the variation of horizontal branch/red clump magnitude with metallicity and age is of order \pm 0.05 dex , which can be neglected in comparison to the intrinsic scatter in the method .
The results are discussed in the context of abundance determinations for red giants in Local Group galaxies .