Re: [ATM] Astigmatism

["Bob May" <rmay@nethere.com>]

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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