code: def sis_sigma_v2theta_E(self, v_sigma):
          
          theta_E = 4 * np.pi * (v_sigma * 1000./const.c)**2 * self.D_ds / self.D_s / const.arcsec
          return theta_E
docs: converts the velocity dispersion into an Einstein radius for a SIS profile
          :param v_sigma: velocity dispersion (km/s)
          :return: theta_E (arcsec)