Are Fresnel rings the same as Diffraction rings?

You could say that. Fraunhofer diffraction, which is at work when we
star test a telescope, is an idealized case of Fresnel diffraction. In
Fresnel diffraction, a spherical light wavefront encounters an obstruction
(such as the circular opening of our pupil). In Fraunhofer diffraction,
the wavefront is flat. In truth, a star does of course emit a basically
spherical wavefront, but the star is so far away that by the time it
reaches our puny little telescope, it is as good as flat.

Fraunhofer diffraction is easier to analyze because of the flat wavefront,
and it gives useful results such as Cor Berrevoets Aberrator program. For
accurate results on nearby light sources, however, one must resort to the
more complex Fresnel treatment.

Qualitatively, they are very similar indeed. Also, I am not responsible
for any change in terminology--these names are much older than I am.

Brian Tung
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