HAWK-I@VLT distortion map for K_s filter. Left: residual trends for the four chips when we use uncorrected stars’ positions. The size of the residual vectors is magnified by a factor of 500. For each chip, we also plot the single residual trends along X and Y axes. Units are expressed in HAWK-I raw pixel. Right: residuals after our polynomial correction is applied. The size of the residual vectors is now magnified by a factor 5000.