PSF anisotropy correction: the first three panels show the ellipticity pattern (measured, derived from the PSFex model, and corrected (e_aniso); X and Y are in pixels). The scale is given by the arrows in the middle (e = 0.1). In the last panel, blue and red dots are the values of e_aniso derived from real stars and from the PSFex model, respectively; green dots are the values after the correction.