Re: ATM Making hole in old mirror.

[astillma@nit.AirTouch.COM (Anthony Stillman)]

Jonas,

Regarding making a hole.  While, I never started coring a prabaloid afer it 
was done.  I've always cored to near the surface before polishing and figuring, 
then finish coring after figuring.  I can tell you about flats. 

Recently I cored three flats from a small 0.5 inch thick sheet of ULE which
was 1/10 over the sheet. I used 220 initially and then went to 360 to punch
through. (Earl Watts taught me to use fine grit to avoid chipping.) After 
coring I tested against a 6" master.  There were still 1/10 except for the very
edge which I beveled anyway.  Then out of curiosity, I tested the slab.  Where
the cut to slab edge was thin the surface was shot, but the remainder of the 
ULE was still 1/10 wave.  I was supprised, as I had heard the same thing, namely
that coring releaves strain and induces strain, there by distroying the figure.

I'm not sure if this is because I cored slowly and didn't let the ULE get hot,
or because it was ULE.  I've not tried this on pyrex, old or new.  I'm certain 
that if the glass were strained before hand things would be different.  But I
have to wonder if this isn't an example of "Well every one has always done it
that way so that must be the only way to do it."  Alas there seems to be a lot 
of that in telescope making.  Of course, some times its true.

Anthony