Re: [ATM] Astigmatism

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

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/
>

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