From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


Basically, when you are able to see stress in the glass, any real grinding
of the glass will change the shape of the glass by the few wavelengths of
light and subsequent polishing will probably produce another wavelength or
so of error.
What is worse is that stressed glass changes its shape with the temperature
due to the changes in the forces inside the glass changing with the size of
the glass (remember all materials change size with temp - usually called
the coefficient of expansion of the material) and this will change the
external shape of the glass over temp.  If the stress was totally radial,
you might be able to get a handle on the problem but stresses are generally
of a random nature and tend to be either point source (tempered glass is
often tempered by a blast of air on spot(s) more than everywhere) or linear
sheet form which is from a running temper process. Just a failure to go
through the annealing temp at a proper slow speed will make for a random
temper as some parts tend to cool slower than other spots and thus the
stress is built up.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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