Contour plots of the spin a and the radial emissivity parameter q. The data were generated with the lamp-post model with the height h = 1.5 r_g. The default value of the spin was a = 0.94, which is indicated by a dashed line in the graph. Different prescriptions for the angular emissivity were used. Top left: angular emissivity from numerical calculations. Top right: limb brightening. Bottom left: isotropic. Bottom right: limb darkening. The χ² values corresponding to the best fit (indicated by a small cross) and to the 1σ,2σ, and 3σ levels are shown in the text legend.