code: def _theta_E_convert(self, theta_E, w_c, w_t):
          
          if not w_t > w_c:
              w_t, w_c = w_c, w_t
          s_scale_1 = w_c
          s_scale_2 = w_t
          f_x_1, f_y_1 = self.nie.derivatives(1, 0, theta_E=1, e1=0, e2=0, s_scale=s_scale_1)
          f_x_2, f_y_2 = self.nie.derivatives(1, 0, theta_E=1, e1=0, e2=0, s_scale=s_scale_2)
          f_x = f_x_1 - f_x_2
          theta_E_convert = theta_E / f_x
          return theta_E_convert, w_c, w_t
docs: convert the parameter theta_E (deflection angle one arcsecond from the center) into the
          "Einstein radius" scale parameter of the two NIE profiles
  
          :param theta_E:
          :param w_c:
          :param w_t:
          :return: