[ATM] Optical flat etc, then what?

[DHA352@aol.com]

Okay I am convinced to test for flatness I need test plate (optical flat)  
that is flatter than my goal. The fringe lines determine how flat my work  is.  
I understand the three work piece simultaneous equation method and its  
limits. Got that much, now some more questions.
 
>From fringe line patterns:
1. How do I tell concave vs convex, or can I?
 
2. What do I do about concave?
 
3. What do I do about convex?
 
4. Chicken-and-the-egg question.  If I need a 1/10delta flat to  test for 
1/4delta, a 1/20delta to test for 1/10delta,...,etc. How is  the flattest optical 
flat tested?
 
A reference to a book that would address some of these questions would  be 
much appreciated.
 
Thanks
Don
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