ATM Trouble w/ slitless Foucault tester

[Barger Kyle <kbarger@haverford.edu>]

I am making a slitless Foucault tester.  Before I put everything together 
for real, I just wanted to put together the light and blade and take a 
look at my mirror to make sure I properly understand how the tester is 
to work.  But I can't really see anything, and I was hoping somebody 
could give me some pointers as to what is wrong.

The mirror itself is a 4.25" f/6.  I have finished fine grinding but have 
not started polishing.  I've measured the focal length at 25" by focusing 
the reflection of the sun.  I am wetting it before I attempt to use the 
tester.

For mock-up purposes, I simply taped an LED (Radio Shack Jumbo Green) on 
a piece of scrap wood near the top, nailed a small piece of wood next to 
it, and taped a single-edge razor blade to the edge of the small block. 
The blade is situated so that it covers half of the LED, and half of the 
blade's length extends above the top of the large piece of wood.

So, when I go to try it out, if I am well inside or outside the radius of 
curvature I can see the reflection of the semicircular LED just great.  
When I am close to the radius of curvature, the image in the mirror is 
difficult to align because the image is large--I assume this won't be a 
problem.  But the real trouble is, when I move the tester to try to 
superimpose the real blade edge and its reflection, nothing happens.  I 
can line things up so there is a small slit, but once I move the tester 
over farther, I don't see any reflection from the mirror.

Am I doing something wrong?  Or is the effect just too transitory to see 
unless I put together the tester for real?