DEEP CCD PHOTOMETRY OF OLD OPEN CLUSTERS

Ages and distance moduli for the open clusters NGC 2204, Berkeley 39, NGC 2477, and Melotte 66 are constrained by comparing the theoretical models developed by Bertelli et al. (1994, A&A, 106, 235) to the obser ved cluster color-magnitude diagrams which are based on deep CCD photo metry. Out of a set of comparison models, no single isochrone was supe rior to the others in describing an observed color-magnitude diagram. Thus, a best fitting model was selected based not only on the match to an observed color-magnitude diagram, but also on the isochrone's agre ement with adopted values for the cluster's metallicity and reddening. The range of otherwise acceptable models help quantify the age and di stance modulus uncertainty of each cluster. Based on the best fitting models NGC 2204 is 1.6(-0.3)(+0.9) Gyr old with (m-M)(0)=13.0(-0.4)(+0 .5), Berkeley 39 has an age of 6(-1)(+2) Gyr with (m-M)(0)=12.9+/-0.2, NGC 2477 is 1(-0.2)(+0.3) Gyr old with (m-M)(0)=10.5(-0.3)(+0.4), and Melotte 66 has an age of 4+/-1 Gyr with (m-M)(0)=13.2-0.1+0.3. (C) Am erican Astronomical Society.