optics

Foucault Knife-Edge Test

My first optics post was more about 3D printing than optics but this one is for real. I have been wanting to see how good the optics in my telescope are so I went about building a Foucault Knife Edge Tester.

A schematic on the Wikipedia article. https://en.wikipedia.org/wiki/Foucault_knife-edge_test#/media/File:Foucault-Test_1.png This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
What my setup looks like. The glow of the red light source can be seen under the camera, the telescope with mirror is at the far end of the table and the knife edge is sticking up in front of the camera.
The heart of the tests, simply a light that goes and a boxcutter blade taped in front. I also added some parchment paper to diffuse the light, and the internet recommended a this configuration where you use a two knife edges to make a slit for the light, I am not sure I could tell the difference. I used one long blade for one side of the slit and the knife edge proper for convenience. This was simply taped to box and a used books to get things aligned well enough. The returning dot just needs to fall some where on the blade and the camera placed behind. It was pretty finicky, a linear stage would be very nice.

It is a simple device which has a point source of light that propagates out spherically and converges back to a small volume after hitting the mirror. Now I say a “small volume” because depending on the shape of the mirror it won’t focus back to an exact point which is how you learn about the mirror! The rays of light hitting the out side of the mirror may converge earlier or perhaps later than other parts of the mirror. The knife edge lets us “cut off” the bundles of light from different parts of the mirror to see how different parts of the mirror are converging light and thus measure its shape.

In my case this didn’t turn out to be that visually interesting in the tester. My telescope happens to be what Astronomy Garage with Reflector has dubbed Bowden Bird Jones/ FC (See a great video on it here) meaning it is a spherical mirror with a sub-aperture corrector in the focuser tube. You can see that corrector in the focuser tube here.

The corrector inside the focuser tube, the reflection is the easiest way to see it.

Because the mirror is spherical (and reasonably good one at that) it does return to nearly a point, meaning that the knife edge either cuts off the light from the edge if it is before of after the focus, or if it is close to the focus generally just cuts it off entirely causing the light to blinking on and off with the slightest touch. This is one of the more interesting results I got:

The image taken in the camera near the focus. We can see the damage around the edges and some scratches on the mirror. Also maybe some texture in the middle of the mirror.

Mostly I saw things like:

Image where the knife edge is in front of the focus point. It is pretty need to see the knife come in from the other side of the image from the knife is physically, a cool consequence of the converging light.

Also to the naked eye, I had a very hard time see much of anything.

I should say my interpretation of the results as a spherical mirror is my understanding given my belief the mirror is spherical given the telescope design and from what I think makes sense. It would be nice to see a parabolic mirror for comparison.

Future Experiments in this Area

  1. I would like to buy a parabolic mirror for the telescope. This would allow me to have a different shape to compare to, but also I am interested in seeing how it compares to the spherical mirror for aberrations with the corrector removed
  2. A linear stage would be a very helpful and perhaps my imprecise setup is preventing me from seeing the shape well. https://gr5.org/bath/ sells linear stages along with the material for a bath interferometer which would also be a lot of fun to try for characterizing the optics of the telescope.
  3. This setup up should also be capable of Schlieren imaging which would be a lot of fun!