First, we analyze the dependence of the FWHM and DOF on the annulus ratio of the annular aperture illumination
under the focusing condition with an oil lens of NA = 1.
2, below, this can be avoided by invoking the theory of partial coherence, or by negating the error in the first place by designing a diffraction experiment so that the aperture illumination is "almost" coherent.
Therefore, it may be desirable to avoid the need for these computations in the first place by designing diffraction experiments so that the aperture illumination will be "almost" coherent and the Fresnel integral (2) can still be used, in spite of the finite size of a given source.
In the archery-target antenna, the phase of the aperture illumination is further controlled by an additional annular reflector, installed in the same plane as the small reflector of a traditional SBFA.
This requires a much wider annular reflector, the shading increases and the overall aperture illumination efficiency is compromised.
However, the aperture illumination
would have to be tapered to reduce the sidelobe levels.
The optional Light Manager adjusts to provide the correct field and aperture illumination
and a constant specimen illumination, regardless of level of magnification.
Its aperture illumination
is switchable by means of ferrite phase shifters so that it can generate a single, low sidelobe beam or three, triple width low sidelobe beams from each one-third section of the aperture, which is called interferometer tracking.