calc_q
======

.. py:function:: luminet.black_hole_math.calc_q(p: float, bh_mass: float) -> float

   Convert periastron :math:`P` to :math:`Q`

   The variable :math:`Q` has no explicit physical meaning, but makes
   many equations more readable.

   .. math::

      Q = \sqrt{(P - 2M)(P + 6M)}

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

   :returns: *float* -- :math:`Q`