Re: [ATM] Pyrex Pie Plate primary

[vorblesnak@peak.org]

> your experiences with the rest of us? Was it worth
> saving the $60?
> 


I have to admit I did not grind the pie plate to save money,  I ground 
it for the challenege.  I have been compiling what I did from the logs 
but was looking for some broader verification.

The glass is very thin, however if you are looking to do an ultrathin 
mirror project this is the place to start.  For 99c you can buy a pie 
plate or pan and grind on the equivilent of 1" of thickness in 60" of 
diameter. I accepted the curve in the bottom and just smoothed it 
out, trying not to change it.  The grind went ..

smooth the bottom with 500 grit, 30 micron, 5 micron,  polish.

I found that the casual strokes we use,  (I use),  to hog out and 
establish the curve on a normal mirror will distort the bottom of a pie 
plate.  Strokes done very slow, very deliberately gave a much better 
result.  

I stiffened the glass with concrete on the first mirror and broke it 
while trying to get the concrete out.  I used plaster on the second 
mirror and was able to pop it out at will.  The cooling of the plaster 
pulled the curve hyperbolic and in the end nothing was gained by 
backing the glass.  I finally figured it out with no backing, mirror on 
top,  classic parabolizing technique.

Those are the high points.  My thanks to Mel Bartels for putting up 
with me while I did it and prodding me to talk about it after the fact.

David Davis
Toledo, OR


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