calc_radius
===========

.. py:function:: luminet.black_hole_math.calc_radius(p: float, ir_angle: float, bh_mass: float, incl: float, order: int = 0) -> float

   Calculate the radius on the black hole accretion disk from a photon's periastron value.

   :Parameters: * **p** (*float*) -- Periastron distance. This is directly related to the observer coordinate frame :math:`b`
                * **ir_angle** (*float*) -- Angle of the observer/bh coordinate frame.
                * **bh_mass** (*float*) -- Black hole mass
                * **incl** (*float*) -- Inclination of the black hole
                * **order** (*int*) -- Order of the image. Default is :math:`0` (direct image).

   .. attention::

      This is not the equation used to solve for the periastron value :math:`P`.
      For the equation that is optimized in order to convert between black hole and observer frame,
      see :py:meth:`periastron_optimization_function`.

   :returns: *float* -- Black hole frame radius :math:`r` of the photon trajectory.