[ATM] Ronchi Readings imitating Foucault

["Russell Jocoy" <russjocoy@hotmail.com>]

To all on the list,

  Most on this list understand that the "Ronchi grating" test defines if we 
have a sphere, a hole in the middle of the disk, a turned up edge, or God 
forbid a turned down edge, and many other surface errors on a large scale. 
Of course this all depends on how far away your measures are from the ROC.
    One point of "Ronchi" testing that baffles most new commers is the shift 
at the ROC of the mirror. This point at the "null" actually imitates a knife 
edge. It really is more of a wire test at that point since the "line" or 
imitated knife edge can not be moved vertically. If it is moved vertically, 
the next line will show the same as the first.  It really does immitate the 
Foucault image  at one point, but since the line does not block out it's 
back or front side on it's "pass" across the image , it just imitates one 
zone near the center..   Am I wrong on this??  You can actually see the 
surface of your mirror with a "Ronchi" grating at ROC , ,Is this a false 
image?, I have seen many surface images that tell good and bad stories.. 
Could these be acurate, and then
  the Foucault testing?     Russ Jocoy      PS  It seems some on the list 
are afraid of Foucault, If they are testing Ronchi at ROC they are so close  
to getting more information with just a vertical move and a knife edge...

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