ATM accuracy of centering primary mirror

[JHM.van.Gastel@psy.vu.nl]

In the atm archives (july '95) I read a contribution of Chris Knight. He 
wrote that his primary was centered within .050 of the tube radius. I don't 
understand this. What does this .050 mean? It cannot mean that a placement of 
a primary has a toleance 5% from the center. Can someone tell me what is 
meant?

I have centered my primary as good as possible. But I do not know if it is 
exactly centered. It is possible that the mirror is placed 1 millimeter or so 
out of center. What is the tolerance? I have a 12 inch f/6 newtonian; tube 
inside diameter 15.2 inch.

Jan