[ATM] 'Commercial' annealed, rather than 'fine' annealed

[artbianconi@blast.net]

In manufacturing, we make no distinction between fine and commercial 
annealing with other materials, at least none I was aware of in over 
30 years of dealing with material properties.

It wasn't until I became involved in mirror making that "going the 
extra yard" and "fine annealing became an issue. 

No where else have I seen such a preoccupation with geometry as I 
have with parabolic mirrors and therein is the probable cause for 
this interest in really slow cooling.

As materials cool, the outer surface shrinks at a higher rate than 
the yet plastic interior. This creates internal stresses. In theory, 
as those internal stresses are relieved, the geometry is altered, 
ever so slightly. And,if you are going out of your way to create a 
defraction limited surface, then any alteration of that surface 
through slow, long term stress relief, is to be avoided.

In the machining industry, we can mechanically stress relieve parts 
by machining the surfaces, all of them. Another method is by 
impacting the surface with anything from steel shot to walnut shells.
It's "sortalike" sandblasting but more aggressive.

Where I suspect the problem is, is that mirrors DO get ground, often 
front and back but mostly just in front. In theory, at least, the 
blank has had a major portion of it's stress envelope removed. If the 
rules for properties of isotropic materials hold true, the reflective 
surface will "age" differently than the rest of the mirror. One need 
not be a physicist to recognize that if two quasi-parallel surfaces 
change their shape at different rates, curvature will set in thus 
altering the mirror in a way that is essentially unpredictable. In 
the rest of the world we don't notice or care about
such a small change. But ATM'ers are dealing in angstroms and 
fractions of a wave so it's perhaps a legitimate concern.

It would appear then that if you are going to grind the front (do we 
have a choice?) we should blanchard grind the back as well.

Mind you, the operative word here is "theory" and the truth is, I 
have never met anyone whose mirror changed it's shape after it was 
figured. At least none they were able to quantify. But then again, 
I've only met a couple of people working on mirrors as large as two 
feet in diameter!

That is one huge chunk of Pyrex!  And, it is going to cost a lot of 
money and effort. Compared to the overall cost of the blank, fine 
annealing is a miniscule increase. You've gone this far. Does it pay 
to start scrimping on what is the single most important element in 
the OTA? I don't think so.

With only a 1 7/8" thickness, I'm curious about the cell design you 
expect to make to keep that thing from flexing

Art Bianconi

===================================

24" x 1 7/8" Pyrex blank from Newport Glass last year - $930
(with Blanchard-ground back), + $90 for a pre-generated curve.
'Commercial' annealed, rather than 'fine' annealed (although I'm not
sure what's actually to be gained from 'fine' annealing).  It was a
'custom' blank, but it doesn't appear that this caused any 
significant increase in the price.

Cheers,
Mike Byorick


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