Re: [ATM] Difficulty Level of figuring 3/4 Plate Glass

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

COO wrote:
> I bought a glass coffee table comprised of several large sheets of
> 3/4 inch thick plate glass at a garage sale this weekend.
> There are 3 sheets 15 X 15 inch square (the legs) and one
> trianglular sheet that is half a 41 X 41 inch square.

Hope it's not tempered glass.  I will assume it isn't for my response.

Check the glass for strain before you start working on it!

> What is the largest mirror one should attempt with 3/4 inch plate.
> Is 15 inches just too big for this thin of glass?  I don't want to
> push any envelopes as others have and are doing, and this will be
> my first thin mirror. :-)

I highly recommend that you grind the back of the mirror blank quite 
flat and support the mirror evenly across the back when you are 
working on it.

I figured a 13.1" F/4.5 plate glass mirror for Russ Jocoy (maybe he'll 
chime in on that) and I had no problems.  The figure is excellent 
(according to my tests and star testing), with no astigmatism.

I also figured a pyrex version of that size mirror for myself, and 
thought it is only 7/8" thick (1/8" thicker than the plate mirror), it 
seemed like it "acted" quite a bit thicker than the plate glass mirror.

The backs of those two mirrors were ground against each other (by 
hand) down to 25 micron to get them really flat.

If you are really serious about high optical quality (I am a bit of a 
perfectionist when it comes to mirrors) I would not go much larger 
than 16" with that thickness of plate.  If you are less concerned 
about having perfect optics (and possibly willing to live with some 
astigmatism) then 20" is quite reasonable.

I have a 22" x 7/8" Pyrex disk that I will make into a mirror 
(probably F/5ish) when I have time.  That will give me more intuition 
into what is possible while maintaining a figure of revolution and a 
superb figure.  I think the 22" will be fine.

I've mentioned these before so I apologize for being redundant.

	Mike Lockwood

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