> Concerning Monochromatic Light Sources for Foucault Testers:
Chromatic abberation by reflection is not the issue. Chromatic abberation of diffraction is the issue. The question is really "does the Foucault test depend on any diffraction effects?" If so, it would probably work better in monochromatic light. I think the answer is probably no, that it is primaraly a geometric test, and this seems to be verified since you always see the test done with white light.
> It is interesting to note that the radius of the Airy disk is also
wavelength
> dependant, so in theory the Airy disk should show some color effects as
well.
> I wonder if any of our intrepid readers has ever seen this?
>
I've asked on numerous occassions what the "correct" size of the pinhole should be for the knife edge test. You want it large to get a lot of light, but not so large that you lose sensitivity. But does it have to be a "point" source. That is, should it be of the order of the size of the airy disk. If so (that's an assumption at this point) then "The airy disk of what?" Is it the size of the airy disk of the whole mirror, or rather the size of the area you are trying to isolate? Say you have a couder mask with one ich openings. Would you use the Airy disk of a one inch aperture? Or perhaps you are trying to see dog biscuit features on the mirror of a quarter inch size. Airy disk for a quarter inch aperture? Intuitively, you would think you would want smaller pinholes for finer structure. On the other hand, since we're working at unit magnification, at the radius of curvature, can you just say "My transverse adjustment on my knife edge is only accurate to .001 inch, so my source can be .001 inch. I believe most pinholes are significantly larger than this, yet work just fine. Or do they? Well, you get the idea. Anybody know what the correct size for the pinhole is? I would like to use a fiber optic for my "pinhole", just need to grab the right fiber. .....RICH