From: "Stathis Kafalis" 
To: 
Reply-To: "Stathis Kafalis" 


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
As you see, there is severe stress visible. Though, I can't see any defects
in the foucaul test. I polished the mirror exactly spherical now and would
see the slightest devialtions.

See for comparison a precission annealed 24" blank here with no shadows visible:
http://www.astrotreff.de/upload/Stathis%20Kafalis/20030605/24zoll_stresstest.
jpg

This question is of great importance for me, as I am seeking a European
source for large blanks that are not too expensive since years and be
wondering how good they have to be annealed.

p.s.
The test was done by placing one photo polarizer filter into a slide
projector and view the light source with a second polarizer filter. The
blank was located in between illuminated by the projector.


Stathis Kafalis
http://www.geocities.com/dobsonstathis

--- BBBS/NT v4.01 Flag-4