Context : 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 \sim 2,000 .
The method for derivation of stellar parameters is presented with application to six reference clusters .

Aims : 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 ( [ Fe/H ] \sim -0.1 ) , NGC 6553 ( [ Fe/H ] \sim -0.2 ) , M 71 ( [ Fe/H ] \sim -0.8 ) , NGC 6558 ( [ Fe/H ] \sim -1.0 ) , NGC 6426 ( [ Fe/H ] \sim -2.1 ) and Terzan 8 ( [ Fe/H ] \sim -2.2 ) .
They cover the full range of globular cluster metallicities , and are located in the bulge , disc and halo .

Methods : Full spectrum fitting techniques are applied , by comparing each target spectrum with a stellar library in the optical region at 4560-5860 Å .
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 .

Results : 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 , T _ { eff } , log ( g ) , [ Fe/H ] and alpha-element enhancements were derived .
We derived < v _ { helio } > = -242 \pm 11 km/s , [ Fe/H ] = -2.39 \pm 0.04 , [ Mg/Fe ] = 0.38 \pm 0.06 for NGC 6426 from spectroscopy for the first time .

Conclusions : The method proved to be reliable for red giant stars observed with resolution R \sim 2,000 , yielding results compatible with high-resolution spectroscopy .
The derived \alpha -element abundances show [ \alpha /Fe ] vs. [ Fe/H ] consistent with that of field stars at the same metallicities .