Age of globular clusters derived from BVRI CCD photometry

The modern generation of electronic detectors, such as the CCD, have made improved photometry possible, especially at magnitudes near photographic plate limits and at wavelengths extending to the near infrared. Vigorous efforts should now be made to extend measurements to longer wavelengths since VandenBerg and Bell (1985) have recently calculated BVRI synthetic isochrones. Consequently, a programme has been embarked on of CCD BVRI photometry for determining the ages of globular clusters. From BVRI CCD frames obtained with the 1.54 Danish telescope at ESO-La Silla, at this moment the reductions have been completed of the following 5 globular clusters: NGC 104 (47 Tuc), NGC 2298, NGC 5139 (ω Cen), NGC 6121 (M4), and NGC 6362. For these clusters ages all close to 17x109y were derived, providing strong evidence that the globular cluster system is coeval and that the epoch of the galactic contraction was short. (author). 4 figs., 5 tabs., 20 refs