>> ...how many rings can you see on a star test? I consistently lose
>> count at around 10.
Is this, like, in focus?! The spherical aberration must have to be
terrible to see so many diffraction rings if that much of the light is
not in the Airy disc (where it's supposed to be). Am I missing something,
or is ring-count actually used as an optical test? In an ideal optical
system, the third ring has only a small fraction of a percent of the total
light (i.e. many stellar magnitudes fainter than the central disc)
so shouldn't be visible at all. Tenth ring?
More generally, the Stans seem to be describing a mix of physiological
and real optical effects, so it's not super-obvious what's going on in
Stan G's original query.
\Brian
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