def derivatives(self, x, y, kappa_ext, ra_0=0, dec_0=0):
        """
        deflection angle

        :param x: x-coordinate
        :param y: y-coordinate
        :param kappa_ext: external convergence
        :return: deflection angles (first order derivatives)
        """
        x_ = x - ra_0
        y_ = y - dec_0
        f_x = kappa_ext * x_
        f_y = kappa_ext * y_
        return f_x, f_y
def derivatives ( self , x , y , kappa_ext , ra_0 = 0 , dec_0 = 0 ) : x_ = x - ra_0 y_ = y - dec_0 f_x = kappa_ext * x_ f_y = kappa_ext * y_ return f_x , f_y
deflection angle

        :param x: x-coordinate
        :param y: y-coordinate
        :param kappa_ext: external convergence
        :return: deflection angles (first order derivatives)