Re: [ATM] Focault Problem

[Mike Lockwood <melockwo@uiuc.edu>]

Kurt,

Kurt Clement wrote:
> 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.
 > What am I doing wrong?
 > Kurt
 > Nixa, MO

There are two possible causes that I know of.

1)  The razor blade may be too close to the LED.  I once had this happen 
when I used scotch tape over the LED, and it was nearly touching the 
knife edge.

2)  I'm not sure what your experience level of testing is.  I want to 
make sure we're clear on the definition of null.  When you say "null out 
the entire mirror", do you mean the entire surface of the mirror goes 
gray at the same time?  Or do you mean that you can get the shadow to 
pass all the way across the mirror until the mirror goes dark?

If it's the latter, then you're not at the ROC, which is why you're 
seeing interference lines (vertical lines parallel to the knife edge). 
If the knife edge is on the right and the shadow enters from the right, 
then you're inside of the ROC.  If the knife is on the right and the 
shadow enters from the left, then you're outside the ROC.

	Mike Lockwood
	Champaign, IL

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