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Re: [ATM] Astigmatism
"We" as an atm community went for thin blanks at the suggestion of John
Dobson because they were readily available for cheap in the 50's when a
large number of ships were being retired after the war. They were portholes.
Lightweight, they made moving telescopes easier. Cooling benefits were
noticed later. Thin also works well for balance of the sidewalk type
telescopes.
As for myself, I am content with full thickness glass and a heavier design.
My next telescope is not likely to be movable. It will sit happily in the
Mountains of New Mexico for it's useful life surounded by pinon trees,
aligator bark junipers, bears and cats.
Francis J. O'Reilly
----- Original Message -----
From: "David Harbour" <stainless_steel@suddenlink.net>
To: "Bob May" <rmay@nethere.com>; <atm@atmlist.net>
Sent: Wednesday, May 21, 2008 10:39 AM
Subject: Re: [ATM] Astigmatism
> 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
>>> _______________________________________________
>>> ATM mailing list http://www.atmlist.net/
>>>
>>
>> _______________________________________________
>> ATM mailing list http://www.atmlist.net/
>>
>
>
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