[ATM] Focault Problem

[Kurt Clement <kurt.clement@flashmail.com>]

I am having problems getting a clear image on the Focault test.

I am using John Upton's modular tester design
http://www.atm-workshop.com/tester-head.html
and have no problem with the Ronchi test using a 100 lpi grating.

The tester was built using a 2.5mm green LED for the test light source.  It 
is an opaque LED and I did not file or sand the front flat.  When I switch 
to the Focault test knife edge (which uses part of a razor blade mounted so 
that it nearly touches the LED), I can get the shadow of the knife edge to 
null out the entier mirror at the ROC.  The problem is that I am seeing a 
large number of vertical lines paralell to the knife edge.  If I tilt the 
knife edge slightly, the lines tilt, so I know it is not the mirror or my 
eyeglasses causing the problem.  The mirror is a 6" f7.5.

With the Ronchi grate, if I get things just right at the ROC I can see a 
shadow profile similar to what I expect with the Focault as I move the 
grating very slightly.  Is the grating acting as a slit and knife 
edge?  The image I get at the ROC by manipulating the Ronchi grating is 
what I expect to see with the knife edge, but is much clearer, and free of 
the the vertical lines I see from the knife edge.

What am I doing wrong?

Kurt
Nixa, MO   

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