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FORS2/VLT survey of Milky Way globular clusters I. Description of the method for derivation of metal abundances in the optical and application to NGC 6528, NGC 6553, M 71, NGC 6558, NGC 6426 and Terzan 8

(abridged) We have observed almost 1/3 of the globular clusters in the Milky Way, targeting distant and/or highly reddened objects, besides a few reference clusters. A large sample of red giant stars was observed with FORS2@VLT/ESO at R ~ 2,000. The method for derivation of stellar parameters is presented with application to six reference clusters. We aim at deriving the stellar parameters effective temperature, gravity, metallicity and alpha-element enhancement, as well as radial velocity, for membership confirmation of individual stars in each cluster. We analyse the spectra collected for the reference globular clusters NGC 6528, NGC 6553, M 71, NGC 6558, NGC 6426 and Terzan 8. They cover the full range of globular cluster metallicities, and are located in the bulge, disc and halo. Full spectrum fitting techniques are applied, by comparing each target spectrum with a stellar library in the optical region at 4560-5860 A. We employed the library of observed spectra MILES, and the synthetic library by Coelho et al. (2005). Validation of the method is achieved through recovery of the known atmospheric parameters for 49 well-studied stars that cover a wide range in the parameter space. We adopted as final stellar parameters (effective temperatures, gravities, metallicities) the average of results using MILES and Coelho et al. libraries. We identified 4 member stars in NGC 6528, 13 in NGC 6553, 10 in M 71, 5 in NGC 6558, 5 in NGC 6426 and 12 in Terzan 8. Radial velocities, Teff, log(g), [Fe/H] and alpha-element enhancements were derived. We derived abundances for NGC 6426 from spectroscopy for the first time. The method proved to be reliable for red giant stars observed with resolution R ~ 2,000, yielding results compatible with high-resolution spectroscopy. The derived alpha-element abundances show [A/Fe] vs. [Fe/H] consistent with that of field stars at the same metallicities.