of this algorithm to the M 13 field shown in Figure 1. We first select all stars inside the estimated tidal radius (25.2$^{'}$; Harris 1996), shown as the innermost green circle in Figure 1. This is regarded as the cluster region. The red dots represent stars with available photometry from the SDSS PHOTO pipeline (Lupton et al. 2001); the black dots are stars with photometry obtained from DAOPHOT. The blue open circles indicate stars with available spectroscopy. We then choose an annulus outside the cluster region, indicated on the Figure as the region between the two black circles, as the field or background region.

We next obtain CMDs of each region, spanning $-1.0 \le (g-r)_0 \le 1.5$ and $12
\le g_0 \le 22$, and then subdivide these diagrams such that the size of each sub-grid is 0.2 mag wide in $g_0$ and 0.05 mag wide in $(g-r)_0$ color. The total number of sub-grids for the CMDs in each region is thus 2500 (50$\times$50). Figure 2 shows the resulting CMDs of the cluster (left panel) and field (right panel) regions, overplotted with squares representing the selected sub-grids, obtained as described below.

We first calculate the signal-to-noise ($s/n$) in each preliminary sub-grid 