def mass_3d_lens(self, R, Rs, theta_Rs):
        """
        mass enclosed a 3d sphere or radius r
        :param r:
        :param Ra:
        :param Rs:
        :return:
        """
        rho0 = self._alpha2rho0(theta_Rs, Rs)
        m_3d = self.mass_3d(R, Rs, rho0)
        return m_3d
mass enclosed a 3d sphere or radius r
        :param r:
        :param Ra:
        :param Rs:
        :return: