code: def function(self, x, y, kappa_ext, ra_0=0, dec_0=0):
          
          theta, phi = param_util.cart2polar(x - ra_0, y - dec_0)
          f_ = 1./2 * kappa_ext * theta**2
          return f_
docs: lensing potential
  
          :param x: x-coordinate
          :param y: y-coordinate
          :param kappa_ext: external convergence
          :return: lensing potential