Differences between the star second moments and their expectations from the spatial smoothing, as a function of f_max (top) and color (bottom). We have selected stars with 3 < IQ₂ < 4 pix², in the z band. The color range was chosen so that the color dependence is roughly linear. One can see that the slope of the top right plot is slightly steeper than the top left plot, reflecting the anisotropy of pixel covariances in flat fields.