Analytical implementation of binning with pixels of size ℓ in the computation of the average structure function. The turbulent velocity field and the cluster emissivity of type Xbeta are both identical to Fig. D.3. Plain lines show exact results using the stationary, unbounded (R = ∞) velocity field, effectively replacing P_2D by in Eq. (12). Dots are obtained by combining the correction formula Eq. (D.4) for a circular domain of radius R = 250 kpc, with the prescription P_2D → P_ℓP_2D. Since the latter is only valid for slowly-varying flux maps, it fails to reproduce the true structure function if pixels are of a sizeable length with respect to R.