Mean ridge line fitting procedure using rotated histograms. This shows the analysis of NGC 1851. In short, the mean ridge line is determined in two steps. First, color histograms are computed for stars grouped in 0.4 mag ( F814 W) wide bins (left panel). A preliminary mean ridge line is then derived with ( F606 W − F814 W) colors set at each histogram's maximum, and the F814 W magnitude of the bin center. Second, we use rotated histograms, perpendicular to the preliminary mean ridge line, to derive the final one (right panel). See text for details.