1997A&A...323..374A

We present new BV photometry for 6564 stars in the field of the globular cluster NGC 6366, using a wide field CCD camera at the prime focus of the Isaac Newton Telescope (INT 2.5m). The mean internal accuracy of the present V magnitudes and (B-V) colours ranges from 0.005mag for the stars at the top of the Red Giant Branch to 0.07mag for the faintest measured Main Sequence stars. From the resultant Colour-Magnitude Diagram (CMD), and considering spectroscopic determinations of [Fe/H] from previous works, we derive the reddening (E(B-V)=0.70±0.04), the distance modulus (m-M_o=12.26±0.15) and the age (18_–3⁺²Gyrs) of the cluster by comparing with the theoretical isochrones computed by Straniero & Chieffi (1991). The difference in V magnitude between the Turn-Off and the Horizontal Branch ({DELTA}V^TO_HB) amounts 3.51±0.15mag. We compare the CMD of NGC 6366 with the CMD of other clusters with similar metallicity in order to estimate relative age differences. We extract two concluions from our analysis (1) NGC 6366 seems to be coeval with the bulk of metal-poor halo clusters, and much older (4-6Gyr) than other metal-rich globular clusters (Pal 12, 47Tuc and M71). (2) On the basis of the comparison between {DELTA}V^TO_HB-age and {DELTA}(B-V)_SGB-age relations, there seems to exist a difference in the Helium abundance between the disk globular system (Y∼0.28) and the (inner) halo globular system (Y∼0.23).