Re: [ATM] Astigmatism on thin blanks

["Joe Mirando" <emohbee@gmail.com>]

Dave,

There are a lot of reasons that people decide to try large, thin mirrors...
Conserving space and/or weight comes to mind. So does conserving cash. But
perhaps the most important reason in a community of crotchety
do-it-yourself-ers, is because the general established wisdom says that you
SHOULDN'T.

Back when I got my 16" dia. 7/8" plate glass blank, I did it because it cost
much less than a full-thickness blank. As you may remember, I was able to
grind and polish the pre-generated curve and get a fairly good finish. My
main problem with the figure was that I ground in a turned-down edge. I was
able to reduce the edge while polishing and, had I not ended up with
vertebrae problems, would have ended up with a beautiful mirror (I still
might, as it sits in my cellar along with all of my grinding/polishing
supplies).

I was concerned about astigmatism, so I took great care to make sure of a
few things. Some of these things were learned here on this very list, some
were passed on to me by people like yourself, and some I tried just for the
heck of it.


   - I did the usual constant turning of the mirror and the tool while
   grinding.
   - I wasn't shy about hacking off edges of the tool where it seemed the
   tiles were getting too thin. (I had one break off and scratch my mirror.
   - I made sure that EVERYTHING stayed wet while I was working... thin
   plate glass seemed like an invitation to heat-induced problems.
   - I was careful to mate the polishing lap to the mirror often.
   - I used a piece of 'astro-turf' welcome mat on top of several dozen
   sheets of wet newspaper while fine grinding and polishing to avoid that
   3-point support you mentioned. The hundreds/thousands of plastic 'grass'
   blades were springy enough to do the trick,

Although I did not get to finish polishing/figuring my mirror... yet... I'm
glad to be able to tell you that there is no astigmatism.

I was going to write "no sign of astigmatism", but that wouldn't be
accurate. When testing the mirror, it was thin enough that I had to devise a
system of teflon blocks on springy material to support the mirror on it's
'backboard' while Foucault testing. Once I was able to get rid of the
unwanted stresses, the mirror through the Foucault tester was a thing of
beauty. My tester, by the way, was loosely based upon your 'Old Number
Five', which you were kind enough to lend me. For some reason, someone even
decided to put a picture of it up on Stellafane's site... never did
understand why.

Anyway, my favorite reason for trying a large, thin mirror was because it
flew in the face of the establishment.

Bob Royce, a member of the local astronomy club and professional glass
pusher, openly scoffed when I mentioned that I was working a 16", 7/8" thick
piece of plate glass. "Heh.. don't even waste your time." he said, like the
seasoned sage that he was.

"Heck," I replied, "ANYONE could do it with full-thickness Pyrex... where's
the challenge in THAT?"

Yeah, I knew that not everyone could do it, and I knew that it was a
challenge regardless, but it felt good to see Bob slowly begin to shake
because I dared to buck conventional wisdom. Had my neck not given out
(arthritis, bone spurs and herniated disc), I might have even finished the
mirror and ended up with an example for those who say, not that it cannot,
but that it should not, be done.


Be well,

Joe
-- 
I Think, Therefore I am... I think
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