ATM Dahl Null Test Question

["Bertapelle, Ken" <Bertapelle.Ken@tci.com>]

Does anyone know anything about a technique called the 
"Dahl null test" for testing the shape of a primary 
mirror?  (I don't know if that's the correct spelling.)
The basic idea, I think, is to use a lens of some sort
in the optical path of the Focault test.  Doing this, I
think, means that the mirror figure is correct when a
null shadow is observed on the mirror, rather than the
zoned shadows of the Focault test.

If anyone can talk more authoritatively about this I'd
appreciate it.  What kind of lens do you use?  How do 
you set up the equipment?  What exactly do you look for?
Is this an easier/better way to test mirror shape?  Does
someone know of a write-up on this technique?

TIA,

- KenB