calc_redshift_factor
====================

.. py:function:: luminet.black_hole_math.calc_redshift_factor(radius, angle, incl, bh_mass, b)

   Calculate the gravitational redshift factor (ignoring cosmological redshift):

   .. math::

       1 + z = (1 - \Omega b \cos(\eta)) \left( -g_{tt} - 2 \Omega g_{t\phi} - \Omega^2 g_{\phi\phi} \right)^{-1/2}

   .. attention::

      :cite:t:`Luminet_1979` does not have the correct equation for the redshift factor.
      The correct formula is given above. The resulting images of the paper are correct though.