Re: [ATM] Astigmatism

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

Bob-

I know that you guys have worked out this astig. monster for thin blanks 
over the years, and rather brilliantly. I have a rhetorical question:  why 
did we, as the atm-ing community, go kind of crazy over these extra thin, 
large blanks? As a proposal, why not just not worry about it, and use a more 
traditional thickness ratio for the blank? (I am anticipating your answer)- 
still would like to see your short version in print- thanks,

Davey


----- Original Message ----- 
From: "Bob May" <rmay@nethere.com>
To: <atm@atmlist.net>
Sent: Tuesday, May 20, 2008 6:04 PM
Subject: Re: [ATM] Astigmatism


> Ah, just been dealing with this fun topic at the glass grinding
> group!
> The thing that happens with thin glass is that it bends.  It
> bends a lot more than full thickness glass does.  As a result,
> when you grind, you end up making a nice spherical surface on the
> glass when it is sitting there but when you lift it off of the
> turntable, it relaxes to a new shape and you realize you're in
> trouble!
> The problem is that you think that the glass is sitting on a flat
> surface and that it is fully supported on that flat surface.
> This really isn't true as there will be three spots that will be
> a little bit higher than the rest of the flat surface and thus
> the glass will be sitting on them.  As a result, you'll be
> grinding in some odd shape to the glass.  The proper solution is
> to provide a reslient surface for the glass to sit on and then it
> will tend to only deform from the tool sitting on top of it.
> Can't have your cake and eat it too!
> One thing is to regularly move the glass about on the turntable
> so that it never gets to the point where the oddities end up
> getting strong enough to matter.
> With coarse grinding and even most of the fine grinding, don't
> bother to do this as the shape is constantly moving about each
> time you're putting the glass down for a grinding session.  In
> addition, the amount of glass being removed at the larger grits
> is so much that it swamps out the errors being ground into the
> glass by the poor support.  I'd be more particular when it comes
> to the last grit before polishing to gain a better spherical
> surface.  All during polishing you will be wanting to regularly
> turn the glass on the turntable so that astigmatism doesn't show
> its ugly head.
> Another thing with thin glass is that you have to properly
> support it when testing or you will be seeing a lot of bending of
> the glass which you will misinterpet as being actually in the
> glass surface and thus will try to get rid of it, finding that
> either you can't (because it isn't in the surface) or polishing
> in the error!
> Hope this makes things a lot clearer for you.
> Bob May
>
> rmay at nethere.com
> http: slash /nav.to slash bobmay
> http: slash /bobmay dot astronomy.net
>
> ----- Original Message -----
> From: libor nemec <liborec.n@seznam.cz>
> To: <atm@atmlist.net>
> Sent: Tuesday, May 20, 2008 2:02 AM
> Subject: [ATM] Astigmatism
>
>
>>
>> Hi guys,
>>
>> I would like to ask if you have any experiences with machine
> grinding of
>> thin mirrors. I try to get into that for a while and I have
> problems
>> with astigmatism when creating bigger diameters (16", 1" thin
>> borofloat). I read somewhere that it is not possible to use
> machine for
>> thin mirrors. Can anyone confirm that?
>>
>> Can you explain the mechanism how the mirror start to be
> affected by
>> astigmatism when a machine grinding is used? And, whats more
> important,
>> how it can be prevented?
>>
>> I have to say that my machine has only one excentr. I use full
> size lap,
>> TOT, 1/3 strokes. I parabolize 1/3 and 1/2 polish lap, TOT, W
> strokes.
>>
>> Thanks,
>> Libor
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>>
>
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