The black circle with radius \(R_s\) represents the event horizon of the black hole.
Photons arrive from the right (\(x=+\infty\)
and \(y=b\)) with an impact parameter value \(b_{crit}\) that moves them on the unstable orbit\(r_{crit}={3\over 2}R_s\) (sphere of photons).
The figure is plotted with Cartesian coordinates \(x=r\cos\varphi\) et \(y=r\sin\varphi\),
considering a black hole of the mass
of the sun \(M\odot\) that is \(R_s=2 \ 953\ m\), \(r_{crit}=4\ 430\ m\) and \(b_{crit}=7\ 672.73\ m\).
The value \(b=b_{crit}\) is an input data (Excel spreadsheet or Python script).
For a black hole with a different mass \(M\), the plots are kept by applying the scaling factor
\(\frac{M}{M\odot}\).