Re: [ATM] Not quite immaculate

[Mark Cowan <toolontop@yahoo.com>]

Pete,

You could have had a raised area on a lap, that chewed
the edge in only one place before disappearing.  It
can happen anytime the edge of the lap hangs over the
edge of the mirror (TOT), or v.v., for a little too
long or when warm.  There are other causes as well...

Leave it alone, BTW. :)

Best,
Mark Cowan
Salem, OR


--- ArtfulBodger <artfulbodger@earthlink.net> wrote:

> 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
> 
> -- 



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