Re: [ATM] Astigmatism on thin blanks

["David Harbour" <stainless_steel@suddenlink.net>]

Thanks for this reply, Joe.

I know why most go for these thin blanks; they do not have deep pockets. If 
they did, they would just by a big Scope from Frank Melsheimer, and sixty 
acres in the middle of nowhere, somewhere. The conclusion of your remarks I 
know only too well.....

Davey

----- Original Message ----- 
From: "Joe Mirando" <emohbee@gmail.com>
To: <atm@atmlist.net>
Sent: Wednesday, May 21, 2008 10:47 PM
Subject: Re: [ATM] Astigmatism on thin blanks


> 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
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


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