Difference between PSF and aperture photometry for tertiary stars as a function of star magnitudes (with the chromatic trend from Fig. 18 and the average background contamination of apertures subtracted). Black circles display the average of data points in bins of magnitudes. Averages are computed with a clipping at 2.5σ as used in the calibration of SNLS supernovae light-curves (Guy et al. 2010).