  (abridged) Accurate reddenings for Globular Clusters could be obtained by
comparing the colour-temperature obtained using temperatures from
reddening-free indicator (Halpha), with that given by standard
colour-temperature calibrations. From a single 1300 seconds exposure with
FLAMES at VLT2 we obtained spectra centred on Halpha (R=6000, 5<S/N<50) for 120
stars near the turn-off of NGC6752. This set of spectra was used to derive
effective temperatures from fittings of Halpha profiles with typical errors of
about +/- 200 K and reddening estimates with individual errors of 0.05 mag.
Averaging all individual reddenings, a high precision reddening estimate has
been obtained for the cluster: E(B-V)=0.046 +/- 0.005. The same exposure
provided UVES spectra of seven stars near the red giant branch bump (R=40000,
20<S/N<40) that provided Fe abundances with internal errors of 0.026 dex, and
with average metallicity [Fe/H]= -1.48 +/- 0.01 +/- 0.06 dex (random +
systematic). Abundances were obtained for several other elements, allowing e.g.
an accurate estimate of the ratio between the alpha-elements and Fe
([alpha/Fe]=+0.27 +/- 0.01). The O-Na anticorrelation is evident from our UVES
data, in agreement with past results. This analysis shows the power of FLAMES
for analysis of globular clusters: the accurate reddenings and metal abundances
obtained by a procedure like that described here, combined with distance
determinations from cluster dynamics or main sequence fitting, and high quality
colour-magnitude diagrams, could allow derivation of ages with errors below 1
Gyr for individual globular clusters.
