[ATM] Testing Tunnel

[G Gagne]

Living in Florida means no cellar for testing mirrors so it had to be 
done in my garage. For testing my 12 inch mirror I purchased a 14 inch 
inside diam., 12 foot long "sonotube" used for pouring concrete columns. 
Cost was less than $50, delivered. I cut the tube to proper length and 
layed it across sawhorses, mounting the mirror at one end and the 
Foucault tester at the other. Being thick rolled paper and closed at the 
end by the mirror, it is extremely rigid, insulated,  and was completely 
devoid of air currents. Never any sign of  vibrations either.
I recently needed to test a 16 inch mirror for a friend but  the 
sonotube was too small. Instead, I set a 14 foot long, 4x6 wood beam 
across the sawhorses to support the mirror stand and Foucault tester. 
The slightest movement of the Foucault tester resulted in the most 
annoying vibrations in the images. The worst vibrations occured with 
Ronchi testing at the center of curvature with just one band showing. 
Some of the vibrations may have been air currents as they continued long 
after the apparatus should have been at rest.

Gil Gagne
Port Orange, FL