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Re: [ATM] Astigmatism
But he didn't, originally, propose blanks less than one inch thick, and over
20 " diameter, simultaneously in the same blank. A very daunting challenge.
I corresponded (typewriter and paper) with John in '77; he was coming my
way, and offered me a porthole glass; what a generous man he was. I was not
in a position at that time to work it, so I declined. But shortly
thereafter, I built #1 Foucault tester. The rest is history; number 4, a
MacIntosh lever arm tester (Y axis readings were via a greatly magnified
movement by having a long rod on a pinion, one end hooked to the Y movement,
and the other, five times as long as the connection with the Y axis tester,
moved along a curved and graduated arc; I lost my beautiful photograph of
Theresa holding same; but after I had been using it for some time, I set it
on the counterweighted (I thought I had put the block in the box) extension
to my first large optical bench, and down she went, and exploded in a
beautiful eruption, cascade of parts. How handy it was that #5, my best, was
ready- all I had to do was go in and get it, and the customer was right
there, and he was impressed!
Someone kept #5, which was my "loaner" to anyone who wanted to borrow it; I
kept it in circulation for a while, but it finally disappeared, as I knew it
would, before I put up the page on it. Perhaps the page is still here.
By the way, I successfully parabolized, for a customer, a 20.5" 1.38"
thhickness mirrror, long ago. No astigmatism. Did it with magic. Here is a
link, put up, as you can see, by Matt Consodinem, try to see if you can find
my free loaner tester page: (I haven't visited it yet, but am, after I take
care of some business about an errata slip; I did make sure to thank Matt.
He is under no obligations to me; how many very fine people, including
F.J.O"Reilly, we have on this list. I could name them, but it would be a
tedious read, for the sheer bulk.
----- Original Message -----
From: "Matt Considine" <matt@considine.net>
To: "David Harbour" <stainless_steel@suddenlink.net>
Cc: <junkwheaton@gmail.com>; <atm@atmlist.net>
Sent: Wednesday, May 21, 2008 10:21 AM
Subject: Re: [ATM] Hex tool page ...
> David,
> FWIW, your other "stuff" is here :
> http://www.atmpage.org/contrib/Harbour/
> Matt
>
Wellness to all-
R-101
----- Original Message -----
From: "Francis J. O'Reilly" <foreilly@bestweb.net>
To: "David Harbour" <stainless_steel@suddenlink.net>; <atm@atmlist.net>
Sent: Wednesday, May 21, 2008 12:08 PM
Subject: 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/
>>>
>>
>>
>> _______________________________________________
>> ATM mailing list http://www.atmlist.net/
>>
>
>
>
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