Foucault Knife-Edge question

I am just about finished with my Foulcault tester and am in the process of adding the Light Soure and the Knife Edge. Does the Knife edge cut into the light beam from the Light Source or the beam returning back from the mirror? Meaning, do I place the Light Source directly behind the Knife Edge or can I put it at the edge of the tester? This will place the Light Source approximately 3.5" away from the Knife Edge.

Thanks

Sam D

Sam, Rick is correct. 3.5 inches is too far, especially if you want to do fast mirrors.

The best thing is to create a so-called "slitless" tester. In these the light source and knife edge are as coincident as you can get them. Do a google search on "Foucault slitless" and you'll find a lot of links regarding how to construct one.

Good luck,
Doug Scobel