From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: 
Reply-To: atm{at}misterg.fsnet.co.uk (Andy Gray)


On Thu, 5 Jun 2003 09:22:01 +0200,  "Stathis Kafalis"
 wrote:

>
>Hello mirrror makers around the world,
>
>how importand ist it, that a mirror blank is free of internal stress? How
>much stress is can be tolerated?
>
>Background:
>I am working on an old 12" mirror of unknown origin. I assume some sort of
>Borosilicate, as the spezific mass is 2,25 g/cm^3 and a white- yellowish tint
>(may be Schott Duran). A polarizer stress test shows following picture:
>http://www.astrotreff.de/upload/Stathis%20Kafalis/20030605/sresstest12zolldur
>an.jpg

Stathis,

Congratulations on rigging up the test. The patterns you see don't look
like a homogeneous material, to me. If homogeneous, but not well annealed,
the pattern would show like the classic "maltese" cross, or
colour fringes over the whole blank. In such a case, I don't see why you
shouldn't achieve a good mirror from it. In this case, it appears that you
can see the pattern that the glass has flowed in as the blank was cast.
This may mean inhomogeneity, and "veins" running across your
polished surface (but since you've already got a polished sphere, why not
continue??).

I don't believe that there is a rigourous definition of "Fine
Annealing", but 10nm retardation / cm of thickness is often quoted for
optical glass. (Actual stress varies because brewster coefficient varies
with glass type.)

Andy

Andy Gray, N. Wales, UK.

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