B and V CCD images of the metal-poor Galactic globular cluster M15 have been obtained, and a complete color-magnitude diagram from the upper red-giant branch to the lower main sequence has been constructed, with homogeneous calibration using the faint M15 standards from Stetson and Harris (1988). Fitting of the CMD fiducial sequence to the Bergbusch and VandenBerg oxygen-enhanced isochrones with an objective statistical method yields an absolute age of 15 +/- 3 Gyr; this is reduced by up to 3 Gyr if isochrones employing He diffusion are used. A differential age determination shows that M 15 is (4 +/- 4) percent older than M92, for a reddening of E(B-V) 0.10 +/- 0.01. A heliocentric distance modulus (m-M)V of l5.40 +/- 0.15 (d = l0.4 +/- 0.8 kpc) is obtained. A luminosity function to M(V) about 8.6 for the cluster main sequence has been derived; the corresponding mass function has a slope consistent with that found in other clusters of similar metallicity and states of dynamical evolution, as well as with values from recent post-core-collapse models of M15.
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Durrell et al. (1993) studied this question.