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Hi everyone,
I have been working a 22-inch blank on-and-off for
too long now, but I encountered a problem recently which took a little time to
solve and correct.
Having completed the polishing sometime ago, I have
since been using a 16½ inch circular lap (used for the initial
polishing) in combination with a 10-inch and 4-inch star laps for
figuring.
I work the mirror face up on a driven base at
about 12 RPM, applying the figuring strokes by hand. Using Ronchi, Foucault
and star tests to evaluate the mirror.
Recently I have been working a broad high zone
at 50-60% radius, using my 10-inch lap to correct it. Ronchi and Foucault
tests seemed to show things going in the right direction, the zone was reducing.
When it appeared to have gone - I star tested.
However, now I found a trace of a line
either side of focus, these being at rightangles to each other, but the
out-of-focus Airey disks appeared circular and even on each side of focus.
Rotating the mirror, rotated the lines but the disks remained the same, so I
presumed the problem was within the mirror.
The only way I found where the
problem was, was to make a series of ring masks. Placing them one at
time over the mirror while star testing until the lines were seen to
disappear.
I used the large lap then after a good pressing to
remove the error.
The only way I found this was to slowly move
through the focus while looking a the ring patterns, had I racked through focus
too quickly to look at the patterns either side I think it would have evaded
detection.
This is just a cautionary note to say that
sometimes (it seems) astigmatism can effect smaller parts of and not just
the whole mirror. Initially I hadn't considered the 50-60% zone the
problem, Ronchi and Foucault looked and read as being correct.
Has anyone else had this problem?
This is the largest mirror I have worked on and I
can't remember anything like this on smaller blanks!
Francis
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