def magnification(self, x, y, kwargs, diff=diff):
        """
        computes the magnification
        :return: potential
        """
        f_xx, f_xy, f_yx, f_yy = self.hessian(x, y, kwargs, diff=diff)
        det_A = (1 - f_xx) * (1 - f_yy) - f_xy*f_yx
        return 1/det_A
computes the magnification
        :return: potential