VI photometry of nearby globular clusters: M3, M5, M13, and M92

(V, V-I) color-magnitude diagrams (CMDs) are presented for the four nearby globular clusters M3, M13, M5, and M92. These clusters span the range -1.3 > [Fe/H] > -2.3, and the new CMDs are intended for use as templates against which to compare the outer halo clusters observed with Hubble Space Telescope F555W and F814W filters. The calibration to the system of Landolt appears to be secure to less than or similar to 1%-2%. Comparing M3 with M13, it seems unlikely that the clusters differ in age by the 2-3 Gyr required to explain the difference in their horizontal-branch (HE) morphology purely as an age effect. If the HE difference is to be attributed to a single agent, we propose that a difference in Y of similar to 0.05 (with M13 having the larger Y) would most economically explain the differences in the main-sequence turnoff and HE morphology between the clusters.