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Re: [ATM] Astigmatism
Bob-
I am dense; but I guess I understand, now, for sure. But why not some exotic
experimenting with cellular built up disks, a little more sweat equity, and
a little less worry about that infernal fatal disease, astigmatism? We need
stiffer mirror blanks, and granted, we need to save money now (I am so out
of touch with the "now" that mirror blank prices are shocking to even me-
Ron Garfron pulled off the deal of the century, when he got my ultra, ultra
fine ground full thickness Corning Pyrex blank, and its too- blank has a
silky smooth fine grind, not a scratch on it, and he has the tool- both for
$200 !!!!
Of course, sixteen is not so large these days. But someone should dedicate
himself or herself to a methodology (I know there is a long, long, long
history of non-success with ribbed, built up disks) but there ought to be a
better way than these large, very flexible glass disks. Heck, I'd be afraid
to handle a large one.
Thanks for straightening me out again, Bob- or rather, for bringing me back
up to date. Remember, I am the crazy fool who gave a nearly full thickness
ribbed commercially made 16" Classical Cass primary to Frank Ward, along
with its 5.37" diameter convex secondary (I had specified wide field, coma
notwithstanding). $4,400.00 from Parks Optical.
Keep me up to date and out of fantasy land, Bob-
Be well, and keep us in line!!!
Davey
P.S. Bob- would a close friend say that it is possible that you are, were,
shall we say, "Depression Scarred"? No shame in that. Now you are trained
for the next one, right around the corner-
----- Original Message -----
From: "Bob May" <rmay@nethere.com>
To: <atm@atmlist.net>
Sent: Wednesday, May 21, 2008 5:44 PM
Subject: Re: [ATM] Astigmatism
> Go down and price a piece of pyrex in a large size of full
> thickness and compare that to a piece of plate glass 3/4" thick.
> Considering that most people that have big telescopes rarely use
> them at their max. magnification, it really isn't necessary to
> have a perfect mirror. Look at Dobson's telescopes for quality,
> look at the old Coulter telescopes and tell me that they're built
> to handle a limiting magnification! A 20" telescope should be
> able to handle a little more than 1000X but think of how
> difficult it is to keep track of anything without a good mount -
> the Az-Alt type mounts where you push the scope around just don't
> hack it! Besides, the probability of finding a place where you
> can see anything through the muck of the atmosphere with such a
> large scope is quite slim.
> Then there's the group of tightwads like me. Got a bunch of free
> glass from a guy that got tired of his glass endtables so he gave
> them to me. Two pieces each of 26", 22" square glass and 4
> pieces of 11"x22" glass for a few gallons of gas to pick them up.
> Bob May
>
> rmay at nethere.com
> http: slash /nav.to slash bobmay
> http: slash /bobmay dot astronomy.net
>
> ----- Original Message -----
> From: David Harbour <stainless_steel@suddenlink.net>
> To: Bob May <rmay@nethere.com>; <atm@atmlist.net>
> Sent: Wednesday, May 21, 2008 7:39 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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