  The visualization of an exceptional point in a PT symmetric directional
coupler(DC) is demonstrated. In such a system the exceptional point can be
probed by varying only a single parameter. Using the Rayleigh-Schroedinger
perturbation theory we prove that the spectrum of a PT symmetric Hamiltonian is
real as long as the radius of convergence has not been reached. We also show
how one can use a PT symmetric DC to measure the radius of convergence for non
PT symmetric structures. For such systems the physical meaning of the rather
mathematical term: radius of convergence, is exemplified.
