NGC 6552 H₂ excitation diagrams of the three extracted regions (top: nucleus, middle: circumnuclear, and bottom: central). The logarithm of the column densities divided by the statistical weights of the different transitions are plotted against the upper level energies of those transitions. The LTE and fitted-OPR column densities are both displayed: the blue circles are the original column densities with LTE g_u, assuming OPR = 3, while the black triangles represent the column densities calculated using the fitted OPR (see Sect. 5.3 for details). The H₂ temperatures, column densities, and masses are listed in Table 4.