Diagnostic plot for the technique. (a) Fit to the stellar mass surface density observed in Nube (the blue symbols) assumed to reside in a Schuster-Plummer potential. The red thick solid line represents the least-squares best fit with an unconstrained DF f(ϵ), whereas the other thin lines are the fits derived from the MCMC exploration of the posterior forcing f(ϵ) to be ≥0∀ϵ and beginning the exploration from the f(ϵ)≥0 least squares solution. These other fits are color coded according to the innermost slope of the surface density profile (ω in Eq. (27)), which always happens to be negative but close to zero. (b) DFs required by the best fit and by the fits forced to have f(ϵ)≥0. Note how the best fit requires f(ϵ) < 0 and so is unphysical. The color code is the same as in (a).