code: def hessian(self, x, y, kappa_ext, ra_0=0, dec_0=0):
          
          gamma1 = 0
          gamma2 = 0
          kappa = kappa_ext
          f_xx = kappa + gamma1
          f_yy = kappa - gamma1
          f_xy = gamma2
          return f_xx, f_yy, f_xy
docs: Hessian matrix
  
          :param x: x-coordinate
          :param y: y-coordinate
          :param kappa_ext: external convergence
          :return: second order derivatives f_xx, f_yy, f_xy