[ATM] Not quite immaculate

[ArtfulBodger <artfulbodger@earthlink.net>]

So I've just finished my first mirror from a 10" Pyrex blank.  I 
enjoyed it (and mirrors number two and three are already in the 
works).  With the help of the guys in the local club, I was able to 
complete the job without too many missteps, and overall the mirror 
tests out nicely.

But.

Using a computerized Foucault tester, I checked the mirror on various 
axes.  It checks out well here; and it checks out well there; and 
it's pretty sweet on this axis, too ... but right here, at this one 
spot, it tests poorly.  Just on the one axis, it shows a major turned 
edge.  It's not garden-variety astigmatism: an inch to either side, 
there's no problem.

Consensus among those with some experience is that I should leave it 
alone.  I don't like the idea, but I'm taking their advice.  I don't 
want to screw up the rest of the mirror trying to fix an error that 
will likely be invisible in use; and this is just a practice mirror 
anyway.  I'm not too concerned about it.

But I don't want it to happen again, when I get to the bigger mirror 
that I ultimately want to make.  So, here's the question for those of 
you with grinding experience: what might have caused this?  How can 
the mirror look so good elsewhere, but still have this flaw in just 
one spot?

I do have a theory.  Maybe you guys can tell me if it makes sense. 
To correct a deepish hole in the center, I did a careful bit of 
polishing with the tool on top, chordal stroke (pitch lap, cerium). 
This worked well, and then I was able to parabolize without much 
trouble; but possibley I overdid the chordal thing just in one spot. 
  But I dunno.  I was pretty careful.

Might that be it?  Or have you any other suggestions?  I'll 
appreciate your thoughts.

Pete

-- 
Artful Bodger
http://www.artfulbodger.net
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