Foucault Mirror Tester

Foucault mirror tester

The Foucault mirror tester is used to test the shape of telescope mirrors as they are being polished in their final stages of manufacture.  Deviations of as little as 1/20 wavelength of light from a spherical mirror surface can be detected with such a device.   When the mirror has a perfect spherical shape, it will appear uniformly gray when viewed with the tester.  Deviations from the spherical shape will appear as light or dark areas.  The slitless design of this tester reduces the number of potential problems with slit orientations and relative slit positions that affect other types of Foucault testers.  The threaded rods on the above tester move the light source and knife edge together by small distances, which can be measured by reading the dials on the tester.   This allows quantitative determinations of  the difference between the sphere and the desired paraboloid.  The desired shape of a final telescope mirror is a paraboloid, not a sphere. Telescope mirrors with high focal ratios (f/8 and higher) have relatively small differences between these two shapes, but mirrors with low focal ratios (f/4 or less) have very large differences and are more difficult both to make and to test.