Hello, I'd like to make a Foucault tester. I read Stellafane instruction and I'm still not sure how to operate the tester, and have some questions about the parts:
1. Where do I put my eyes? Is it the same as hole (A) where the knife edge is?
6. Nose issue: can I cut out part (B) to solve it? Will that affect the functionality in any way?
I look at Edmunds Optics Foucault tester and they look way basic compare to this yet the price is almost $200 
The Edmund Optics tester is very basic and rather expensive.
It has a light source separate from the knife.
The Stellafane tester has the light source touching the knife edge and they move together. This is the most common form of the tester in recent years, maybe a couple of decades.
As for eye accessibility to the knife, there is no need for the nose blocking wall supporting the light source and knife. You could do away with the peepshow wall and incorporate the Edmund type stalk supporting the knife and light source to your Stellafane tester measuring engine carriage. The light source only needs to be a small light attached to and partially obscured by the knife. A small bright LED with some sort of diffusing material between the knife and LED. White plastic milk carton or similar material makes a good diffusion layer between LED and knife. It could help to have a 1 or 2 mm or so pinhole mask either between the diffusion layer and the knife or on the other side of the knife. Try to minimize the size distance the light source extends behind the knife edge because you need to get your eye very close to the knife.
The knife doesn't need to be razor sharp. For safety if a razor blade is used I recommend you dull it. I sometimes use a razor that I have ground with very fine grit, 5u or 9u against a flat plate glass (window pane). Holding the blade at nearly a right angel to the glass pane, it only takes 30 seconds or so to dull the edge. Works just as good, maybe even a little better than a razor sharp edge. It can help reduce reflected light from the knife if you paint it black on the back side. You might want to rub the edge again on the glass to remove any paint that might be on the very edge.
Consider which eye you will use and put the knife edge on the side of the support stalk that will allow that eye easiest access to the knife.
Make it all as comfortable to use as possible. To do good testing you will be spending some time with your eye up to the tester.
Edited by dogbiscuit, 19 September 2021 - 10:30 PM.