Re: [ATM] Astigmatism on thin blanks

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Russell Jocoy wrote:
> 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...........

Yes, I remember that glass.  Here's a link to a nice looking scope:
http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/jocoy/jocoy13.html

Russ, did you fix the secondary yet?

Peter wrote:
> 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.

A machine can (and really should) produce a figure of revolution.  I 
polished the 13.1"x0.75" and I regularly polish 16.5"x1.2" mirrors on 
machines without introducing measurable astigmatism.

> 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.

I agree with a all of that, except for the fire-hydrant part.  Sent it
to someone who can anneal it instead!

> 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.

The rotation is a good insurance policy, and is good practice.

That said, I  want to point out that if the support layer under the
mirror is truly doing its job evenly and the turntable or grinding
stand under that is quite flat, then the rotation is technically 
unnecessary!

If the support layer is not doing its job evenly, every time we rotate
the mirror the support layer will push up (or allow sagging in) a
different part of the mirror, and one relies on only frequent rotation
and averaging to make the mirror a figure of revolution.  This is not
good practice.

The thinner the mirror, the more critical the support.

Mark Cowan's points about the test stand are good ones.  As for
testing, I also don't use a sling for support.  It can be made to work
(as shown by Robert Houdart), but I use padded support pegs, radiused
blocks, etc.  Distortion from poor test stand support is plainly 
visible in a good artificial star test, as is departure from a figure 
of revolution.

	Mike Lockwood



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