Ratio ξ = (S / N) / (S / N_max) for all Lyman α-emitting sources in the Hubble Deep Field South MUSE datacube from Bacon et al. (2015). Here, S/N_max is the maximum S/N_peak value of a source, over all considered filter widths. The grey lines denote the ξ for the individual emitters, while the blue points and connecting dashed curve show the average relation. For a filter width of FWHM_v = 250 km s^-1 (vertical dashed line), almost all Lyman α line emitters have ξ> 90%. The red curve shows the theoretically expected ratio for an assumed Gaussian emission line with FWHM_v,true = 250 km s^-1 filtered with an incorrect template of width FWHM_v = κ × FWHM_v,true.