Magnitude difference between HAWK-I and 2MASS as function of the 2MASS magnitude. The black dots represent all the stars matched between HAWK-I and 2MASS with good photometry. Red crosses show stars from the saturation limit (K_S =−15.46) to two magnitudes fainter. The solid red line is the zero-point (median of the magnitude difference of the red crosses); the dashed line are set at a zero-point ± 1σ (defined as the 68.27th percentile of the distribution around the median). The label in the top left corner gives the zero-point ± σ/, where N is the number of stars used to compute the zero-point.