SB profiles of the group central galaxies beyond 20 arcsec radial distance from the centre for zeroth- (blue), first- (purple), and second- (red) order polynomial background estimations, respectively. In all the profiles, error bars indicate 1σ uncertainties on the mean. All three profiles are the same within 20 arcsec (not shown). Beyond that, however, higher-order background estimations over-subtract the background compared to the lower-order ones. This is the most prominent for the second-order polynomial estimation of the background.