The FWHM variation measured across the field F1 in K_s filter (left) and across the field F2 in H filter (right). The FWHM scale ranges from 0.07′′ to 0.5′′, as is shown by the colour bars on the right. The (black) contours identify iso-density levels with a step of 0.03′′. Left panel: the FWHM is quite uniform varying from 0.15′′ to 0.24′′. This field is well centered between the three guide stars. Right panel: the FWHM is very good (0.09′′) near the guide star GS1 (upper right hand corner), but the correction rapidly declines (down to 0.24′′) towards the opposite corner.