Foucault knife-edge test

The Foucault knife-edge test was described in 1858 by French physicist Léon Foucault to measure conic shapes of optical mirrors, with error margins measurable in fractions of wavelengths of light (or Angstroms, millionths of an inch, or nanometers). It is commonly used by amateur telescope makers for figuring small astronomical mirrors. Its relatively simple, inexpensive apparatus can produce measurements more cost-effectively than most other testing techniques.

It measures mirror surface dimensions by reflecting light into a knife edge at or near the mirror's centre of curvature. In doing so, it only needs a tester which in its most basic 19th century form consists of a light bulb, a piece of tinfoil with a pinhole in it, and a razor blade to create the knife edge. The testing device is adjustable along the X-axis (knife cut direction) across the Y-axis (optical axis), and must have measurable adjustment to 0.001 inch (25 µm) or better along lines parallel to the optical axis. According to Texereau it amplifies mirror surface defects by a factor of one million, making them easily accessible to study and remediation.

The mirror to be tested is placed vertically in a stand. The Foucault tester is set up at the distance of the mirror's radius of curvature (radius R is twice the focal length.) with the pinhole to one side of the centre of curvature (a short vertical slit parallel to the knife edge can be used instead of the pinhole). The tester is adjusted so that the returning beam from the pinhole light source is interrupted by the knife edge.

Viewing the mirror from behind the knife edge shows a pattern on the mirror surface. If the mirror surface is part of a perfect sphere, the mirror appears evenly lighted across the entire surface. If the mirror is spherical but with defects such as bumps or depressions, the defects appear greatly magnified in height. If the surface is paraboloidal, the mirror usually looks like a doughnut or lozenge although the exact appearance depends on the exact position of the knife edge.

It is possible to calculate how closely the mirror surface resembles a perfect parabola by placing a Couder mask, Everest pin stick (after A. W. Everest) or other zone marker over the mirror. A series of measurements with the tester, finding the radii of curvature of the zones along the optical axis of the mirror (Y-axis). These data are then reduced and graphed against an ideal parabolic curve.

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