Optimal bin size Δ for model spectrum binning with a Gaussian lsf, as a function of the number of counts per resolution element N_r. Dash-dotted line: zeroth order approximation (18); thick solid line: first order approximation (19); dashed line: second order approximation (20). For comparison we also show as the dotted curve the approximation (36) for the data grid binning. All results shown are for R = 1.