Maps of the slope parameter (S_OE) for Set 1 (left panel) and Set 2 (right panel) at different wavelengths. The dark blue parts correspond to regions with steeper slope (i.e., smaller S_OE), while the redparts represent a shallower slope (i.e., higher S_OE). The opposition point is indicated by red arrows in the bottom panels. Green dashed lines separate regions where Δ α is >0.°3 (leftward) and <0.°3 (rightward), respectively, suggesting that the data to left side of the line satisfied Criterion 1 (see Sect. 2.2.5).