Re: [ATM] Pinhole size article in ATM 3

["Bob May" <rmay@nethere.com>]

The old folks made a lot of assumptions that really aren't all
that viable.  One of them is the pinhole size.
Since then people have discovered that all you really need is an
edge to do a sharp cutoff of thee light source, ala the slitless
version of the light source, to do the test.  The edge of the
pinhole is the part that actually does the work on the pinhole
while the rest of the pinhole just limits the amount of light
that you see when you are not cutting off the light beam.  All
this does is sets the amount of the light that is avqilable
before the light is cutoff o the desired light level for the
test.  On a perfect world, the light would have a very sharp
change from white to black with no slope between.  In reality,
there is actually a slope from full light to no light and the
size of that light source determines how the upper end of the
slope will act.  In other words, the pinhole of diffraction
limited size (what the old folks were looking for in the small
piinhole size the curve would be fairly linear from white to
black while the slitless tester, on the other hand, the slope at
the white end is quite gentle and gradually increases as the
image gets darker, slowly falling into line with the diffraction
limited pinhole when near the black end of the slope.  This is
why you want to test with a slitless tester near the dark end of
the slope as the light sournce is the smallest and the shadows
are most defined.
Bob May
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