calc_k
======

.. py:function:: luminet.black_hole_math.calc_k(periastron: float, bh_mass: float) -> float

   Calculate the modulus of the elliptic integral

   The modulus is defined as:

   .. math::

      k = \sqrt{\frac{Q - P + 6M}{2Q}}

   :Parameters: * **periastron** (*float*) -- Periastron distance
                * **bh_mass** (*float*) -- Black hole mass

   :returns: *float* -- Modulus of the elliptic integral

   .. attention:: Mind the typo in :cite:t:`Luminet_1979`. The numerator should be in brackets. The resulting images of the paper are correct though.