A BV PHOTOGRAPHIC AND CCD ANALYSIS OF THE INTERMEDIATE-AGE OPEN CLUSTER NGC-3680

New photoelectric and CCD data on the BV system have been obtained in the open cluster, NGC 3680, and used to calibrate a photographic survey of over 490 stars down to V = 17.6 in the field of the cluster. The cluster color-magnitude diagram exhibits a well populated red giant clump, but effectively has no red giant branch. The turnoff region has a sharp cutoff at (B - V) = 0.4, with a red hook and a large amount of scatter, consistent with the explanation of a significant binary population. Comparison with model field star counts for this region of the sky demonstrates that the cluster main sequence is, at best, poorly populated below V = 14.0 (M(v) = 4.0). Adopting E(B - V) = 0.05, [Fe/H] = 0.1, and (m - M) = 10.0, the standard isochrones of VandenBerg [ApJS, 58, 711 (1985)] produce an age of 1.9 +/- 0.5 x 10(9) yr, but appear unable to match the morphology of the turnoff region. The discrepancy is removed when the comparison is made to isochrones which include convective overshoot. An approximate transformation relation between the respective age scales leads to an age of 4.5 x 10(9) yr for NGC 3680 on a system where the Hyades is 1.2 x 10(9) yr old and M67 is between 6 and 7 x 10(9) yr old. Comparison to the color-magnitude diagrams of clusters of similar metallicity indicates that NGC 3680 is intermediate in age to the Hyades and IC 4651, but only slightly younger than IC 4651, consistent with the color distribution of the red giants. The comparisons to both theory and observation imply that the distance modulus for the cluster needs to be increased to (m - M) = 10.3.