Comparison of Sérsic and Spergel indices. The data (crosses) in the top panel represent the galfit Sérsic indices measured for sources created with the input Spergel model for a range of v values from −0.7 to 0.5 and for different source effective radii (expressed in terms of the FWHM of the synthesized beam, R_e/θ_b) ranging from 0.1 to 2.0. The solid curves in the top panel denote the best-fit empirical relation between Spergel v and galfit Sérsic n, depending on R_e/θ_b, as expressed by Eq. (3). The lower panel displays normalized residuals to the proposed relation.