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