burrjaw at earthlink.net
Sun Oct 11 08:34:37 JST 2009
At 2009-10-10 12:26 +0200, Vladimir Galogaza wrote:
>1 nm can be neglected compared to usual amount of p-v errors in figuring and
>polishing. Why then should we bother and abandon HCF use.
>What will 1 nm structured "ripple" do to image?
It might behave like a diffraction grating, so in
some off-axis direction, there would be a maximum.
>But nevertheless, the few last strokes are still strokes.
>Why should hexagon edges be sharp not smeared out.
The HCF hexagonal pattern has 3 sets of parallel
edges 120° apart. If the stroke were along one
of these directions, the maximum action would be
in parallel groves along those edges. Now if you
rotated the mirror by 60°, the stroke would be
along another set of edges, and the mirror would
end up with a pretty hexagonal micro-ripple.
I think it is possible that this explains the
non-hexagonal HCF ripple shown in Tex's Fig.
41C.1 on page 90. A pair of groves show up which
might be the result of the last stroke, but the
rest of the random bumps might be coming from
random rotations of the mirror between strokes.
And it might also be an explanation of why, as
all the ancient experts say, don't complete
figuring with HCF - finish with pitch.
-- Jim Burrows
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