Differences in the measured position of the PSF centroid for the various models used. Top: difference in position in the cross-dispersion direction (δ_⊥). Bottom: difference in position in the dispersion direction (δ_∥). In both panels, the number count axis is logarithmic. The black histogram compares the elliptical Moffat profile to the dual-Gaussian profile. The grey histogram compares the asymmetric-Gaussian profile to the dual-Gaussian profile. The light grey histogram compares the asymmetric-Gaussian profile with the elliptical Moffat profile.