code: def sis_theta_E2sigma_v(self, theta_E):
          
          v_sigma_c2 = theta_E * const.arcsec / (4*np.pi) * self.D_s / self.D_ds
          return np.sqrt(v_sigma_c2)*const.c / 1000
docs: converts the lensing Einstein radius into a physical velocity dispersion
          :param theta_E: Einstein radius (in arcsec)
          :return: velocity dispersion in units (km/s)