def METHOD_NAME(self):
    kwargs_arc = {
        "tangential_stretch": 3,
        #'radial_stretch': 1.,
        "curvature": 0.8,
        "direction": 0,
        "center_x": 0,
        "center_y": 0,
    }
    kwargs_arc_sis = {
        "tangential_stretch": 3,
        "radial_stretch": 1.0,
        "curvature": 0.8,
        "direction": 0,
        "center_x": 0,
        "center_y": 0,
    }
    x, y = util.make_grid(numPix=100, deltapix=0.01)
    f_x_sis, f_y_sis = self.arc_sis.derivatives(x, y, **kwargs_arc_sis)
    beta_x_sis = x - f_x_sis
    beta_y_sis = y - f_y_sis
    f_x_const, f_y_const = self.arc_const.derivatives(x, y, **kwargs_arc)
    beta_x_const = x - f_x_const
    beta_y_const = y - f_y_const
    from lenstronomy.LightModel.light_model import LightModel
    gauss = LightModel(["GAUSSIAN"])
    kwargs_source = [{"amp": 1, "sigma": 0.05, "center_x": 0, "center_y": 0}]
    flux_sis = gauss.surface_brightness(beta_x_sis, beta_y_sis, kwargs_source)
    flux_const = gauss.surface_brightness(beta_x_const, beta_y_const, kwargs_source)
    npt.assert_almost_equal(
        (flux_const - flux_sis) / np.max(flux_const), 0, decimal=2
    )
    # check for stability outside the defined bounds of curvature
    f_x_const, f_y_const = self.arc_const.derivatives(x=0, y=1000, **kwargs_arc)
    npt.assert_almost_equal(f_x_const, 0)
    npt.assert_almost_equal(f_y_const, 0)