Left panel: true-color image composite of I Zw 18 produced from archival HST data, showing the extended nebular halo of the BCD (adapted from Papaderos & Östlin 2012). The horizontal bar corresponds to 5″. Upper-right panel: SL model with iFIT (blue) to the observed R-band SBP of I Zw 18 (SBP1; from Papaderos & Östlin 2012). The inset shows a Moffat model of the PSF (see appendix for further explanations) and the fit residuals are displayed in the lower diagram. Bottom panel: variation of the best-fitting Sérsic index η vs. limiting surface brightness μ_lim obtained by Papaderos & Östlin (2012) for SBP1 and SBP2 using standard χ² minimization (gray dashed and dashed-dotted curve, respectively). Open and filled circles show the result obtained with iFIT by modeling both SBPs down to a μ_lim between 23.5 mag/□″ and 29 mag/□″.