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Stargazing Network
Usage of the Foucault tester started in February. While the mirror was not really ready for testing, it was satisfying to test the mirror and see a dim pattern in the expected form. The room is darkened since the image is quite dim. (Remember, it has not been aluminized yet!) We had to make sure to warn both Jim and John before the flash went off, since eyes need to be adapted to low light levels to see the reflected light.

Jim is seen trying to find the reflection of light from the slit on the Foucault tester.
John is seen adjusting the mirror stand up and down by adjusting the screw in the front of the holder.  By moving the entire cart side to side and the adjustment screw up and down, the reflected image from the slit should be focused back to Jim and the knife edge.
Once the knife edge is properly placed, this is the image that was seen when looking through the tester.  A spherical surface will appear flat.  There is still some surface roughness left since the mirror is not yet fully polished.  The raised ring is due to some problems with the curvature of the prepolishing tools.  Fortunately, this is (very roughly) the general appearance that a paraboloid shows in the test, although it shouldn't be as extreme as in this appearance.   Notice that you see the inside of one ridge on the mirror and the outside of the other.  The bright lines down the centerline of the image are actually reflections from the back of the mirror, we believe.  If you look carefully at the image, you can barely see a pattern of spiral lines.  These are from the periodic strokes of the grinding machine.  Since this is early, this is of little concern.  Later polishing should remove this error.
Second test...