6 Discussion and conclusions

We have carried out VI CCD photometry of about 40000 stars down to V = 23 mag. They are located in an area of about $6 \times 10$ arcmin² in the direction of the cluster NGC 6553. Our photometric zeropoint calibrations agree fairly well with the HST data. Present observations have derived the parameters for the field population present in the direction of the cluster for the first time. These indicate that NGC 6553 is situated behind the young ($\sim$ 800 Myr) population of the Galactic disk but in front of most of the bulge stars. The young population located at a distance of $\sim$ 2 kpc suffers in interstellar extinction of $A_v\sim$ 2.2 mag. This is consistent with the average standard interstellar absorption in the disk of $\sim$ 1 mag/kpc adopted in many studies of galactic structure (cf. Neckel & Klare 1980 and references therein). This indicates that there is almost no interstellar matter present between the young population of the Galactic disk and the cluster, as both have almost similar interstellar extinction with $E(V-I)\sim$ 0.9 mag. This is in accordance with the structure of Galactic thin disk, because, along the line of sight, at the distance of young population, we are leaving the thin disk where the dark clouds are concentrated.

The morphology of the RGB corrected for field star contamination confirms the nearly solar metallicity for the cluster population determined spectroscopically. Tilt and elongation in the HB of cluster population can not be understood in terms of the presence of differential extinction across the cluster region.

Acknowledgements

We gratefully acknowledge the useful discussions with K.S. de Boer and V. Mohan. A. Dieball helped in field star subtraction. The valuable suggestions given by anonymous referee has improved the paper significantly. The financial support provided by the Alexander von Humboldt Foundation to RS is gratefully acknowledged. TR wants to thank the Alexander von Humboldt Foundation for generous support and the Indian Institute of Astrophysics, Bangalore for hospitality and financial support. EKG acknowledges support by the German Space Agency (DARA) (grant 05 OR 9103 0) and by Dennis Zaritsky through NASA LTSA grant NAG-5-3501. Furthermore, we thank S. Ortolani for mailing his ground based CCD photometric data. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.