Background residuals as a function of the contamination index. The contamination index at a given location for a given aperture is defined as the flux in the aperture expected from objects detected in the field, assuming they have a Moffat PSF profile (see Eq. (19)). The top figure displays the average background level (in ADU per pixel) in random apertures as a function of this contamination index. We concentrate on random positions with low contamination index (apertures expected to be mostly empty), sampling the background level far from detected objects. The shape of the curves reflect the remaining structure in the background due to PSF tails. The bottom plot displays the distribution of the contamination indexes at the position of tertiary stars (excluding the contribution from the tertiary star itself). Horizontal lines in the top plot display the average value of the curve weighted for tertiaries, i.e. the expected per-pixel background offset for tertiaries.