Re: [ATM] Astigmatism on thin blanks

["Peter" <peter@kitgear.com>]

This is a very good thread.

I worked an 18.75" x 1" thick plate glass mirror this past weekend. After
machine polishing it had quite a bit of astigmatism. We worked most of it
out by hand using figuring strokes. We should develop a documented method
for this ATM group how to properly work thin glass. Thin glass is our
future.

Here's a start:

I'd start with a reasonable thickness ratio not exceeding 20:1. I've done
25:1 but it was tough and I'd stay away from it.
Stress test the mirror to make sure you have a usable piece of glass.
Toss it against a fire hydrant if you see stress so you're not tempted to
give the glass a try. This is a critical step!
Grind the back flat or a figure of revolution. Non-uniformities on the back
do transfer to the front. Don't guess at this one just do it.
Proper support under the mirror is critical.
Lots and lots of mirror rotation during fine grinding, polishing and
figuring.
Test in a sling.
Use PLOP to design your mirror support system.
Use good edge support (astatic, roller bearing or piano wire) when in the
scope.

Regards,


Peter
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf
Of Russell Jocoy
Sent: Wednesday, May 21, 2008 8:26 PM
To: ATM list
Subject: [ATM] Astigmatism on thin blanks



 To all,   David Harbour made a statement of fixing the astig. problem of
thin glass work is to buy THICKER glass.  ATM"s in the past had no choice
but to use thick glass, and also that thin glass was TABOO... Thus comes an
experimentation situation....1.. Thick glass is heavy, takes a long time to
get itself stable to the outside temp's....time is important to an observer.
2.. The recent rise in cost of "pyrex" or any "thick" glass has hit the
wallet of the ATM hard, as have other cost's.

   ATM's are trooper's and don"t give up that easy.. Thin glass has been
tackled for over 30 years, (ask Mel) Thin glass is not a horror story, and
not a auto astig. situation. The times are a changin'.
  Thick glass is available if you can afford it.. Thin glass is available
and the few who go there need to take care, and read, listen, learn, and not
bitch when you don't follow instructions and end up with a monster, Like Bob
May says, The only way you can destroy a piece of glass is to break it,
anything on it's surface can be changed....unless you give up the goat...
Russ
    Thin glass within reason can produce a beautiful telescope, I own a 13
inch by 3/4" plate glass mirror that produces fantastic images to the
eye...f4.7  pushed yes, but possible...........
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