def D_ds(self, H_0, Om0, Ode0=None):
        """
        angular diameter from deflector to source
        :param H_0: Hubble parameter [km/s/Mpc]
        :param Om0: normalized matter density at present time
        :return: float [Mpc]
        """
        lensCosmo = self._get_cosom(H_0, Om0, Ode0)
        return lensCosmo.D_ds