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Air Currents in the Ronchi and Foucault Tests
The following Ronchigrams and Foucaultgrams were taken at the center of curvature of a 15” spherical mirror. This mirror will eventually be a 15” F/4.5 Parabola.
The three Ronchigrams above were taken inside Rc with a 50 line/inch grating. The image on the left was taken when the air was relatively steady. The middle image was taken shortly after someone walked through the optical path. This shows fast moving turbulent air with relatively small scale structure in the turbulence. This is probably a good approximation of what you would see on a night of poor seeing.
The image on the right was obviously taken with someone’s hand in the optical path. This demonstrates slower moving convective currents where the scale of the turbulent flow is larger. This would be fairly typical of the structure you see while your mirror is cooling down (or if someone is standing too close to your open truss Dob).
The above Foucaultgrams were taken at the center of curvature of the mirror under the same conditions as above. The Foucaultgrams show the scale of the turbulence cells much more clearly. You will also note that, in the image on the left, there is a faint annular zone in the mirror. This zone is completely masked by the air turbulence in the other images. [This mirror was tested after the polishing process, which naturally produces a sphere. The faint annular zone will disappear during the parabolizing process.]
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