def function(self, x, y, kappa_ext, ra_0=0, dec_0=0):
        """
        lensing potential

        :param x: x-coordinate
        :param y: y-coordinate
        :param kappa_ext: external convergence
        :return: lensing potential
        """
        theta, phi = param_util.cart2polar(x - ra_0, y - dec_0)
        f_ = 1./2 * kappa_ext * theta**2
        return f_
lensing potential

        :param x: x-coordinate
        :param y: y-coordinate
        :param kappa_ext: external convergence
        :return: lensing potential