Left panel: color–magnitude diagram of the J-PLUS M15 stars. The population is over-plotted with a PARSEC isochrone corresponding to an age of 10.5 Gyr, and metallicity of Z = 0.0001 (Bressan et al. 2012), with an assumed extinction of E(B − V) = 0.11 mag. Of the 1437 stars in the sample, the 15 with SEGUE-1 medium-resolution spectra are denoted by green triangles. Right panel: distribution of photometric metallicity estimates from M15. A Gaussian distribution was fit to the photometric distribution, with the σ = 0.33 dex region shaded. A stripe density plot is shown for the 66 confirmed M15 members with medium-resolution parameters from the SSPP. The shaded region in the density plot depicts the S_f of 0.13 dex.