Re: [ATM] Astigmatism on thin blanks

[Mark Cowan <toolontop@yahoo.com>]

Hi Peter,

Valid techniques!  If you test with Foucault a sling will do as the test stand
sag is across the horizontal.  But if you want to get lower levels of astig on
the test stand I recommend a different approach.

First the analysis:

The thin mirror folds over because it can.  (OK so far?)
To fold the top and bottom move forward and the center moves back (a tiny bit).
The forces involved are very small and can be defeated easily.

Now the solution:

Flat back on the mirror is a necessity.
Use a test stand with a flat back as well.
Use pegs with rubber hosing (heater hose on dowels) at 90 degrees for edge
support.
Place the mirror against the stand above the pegs, then without inducing any
extra force let it slide gently onto the pegs.  You may need to seat it a few
times to get it right.

Best,
Mark


--- Peter <peter@kitgear.com> wrote:

> 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



      
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