This tool is called a “Foucault Instrument” from french physicist Leon Foucault who amongst many other things invented it in 1858. It is used when you perform a Foucault knife-edge test. This instrument can tell if a mirror has a perfectly spherical shape or if it has non-spherical deviations on its surface.
The instrument I built is based on the design by Jean Texereau from his book “Construction du Telescope d’amateur“. Here’s how the instrument works:
- A light bulb or LED produced a bright light which travels trough a small linear aperture (about 30µm).
- This dash of light gets reflected by the spherical mirror placed in the front of the instrument at a distance which correspond to your mirror’s radius of curvature.
- When the beam of light comes back to the instrument, it should have the same “shape” as when it left it (i.e. a short and thin vertical dash).
- This beam is intercepted by a razor sharp knife. From there, if you place your eye behind the knife, you will be able to tell the shape of the mirror:
- If you see a uniform grey color on the mirror, this means that the beam was equally intercepted by the knife, so it came back straight. You can conclude that the mirror has a spherical surface.
- if you see a rough or non-uniform color on the mirror, it means that the beam did not come back straight and was not intercepted evenly by then knife (some light was totally blocked while some was entirely missing the edge of the knife). The conclusion of this observation is that the surface of the mirror is irregular and need to be carefully reshaped by polishing.