Testing centricity of optical elements using point spread function

It is shown that a specific position of an axial object point can be found for every optical element, where the spherical aberration is either zero or minimal. If we image this point with an optical element, then its point spread function will be almost identical to the point spread function of the diffraction limited optical system. It can be used for testing of centricity of very precise very precise optical elements, because we can simply detect unsymmetry of the point spread function, which is caused by the decentricity of the tested optical element. Furthermore, one can also use this method for testing optical elements in connection with a cementing process. It is also derived a simple equation for calculation of the coefficient of third order coma, which is caused by a decentricity of the optical surface due to a tilt of the surface with respect to the optical axis.