Foucault knife-edge test

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.

[2] The test can measure errors in a mirror's curvature to fractions of wavelengths of light (or Angstroms, millionths of an inch, or nanometers).

A series of measurements with the tester, finding the radii of curvature of the zones along the optical axis of the mirror (Y-axis).

Some telescope makers use a variant of the Foucault test called a Ronchi test that replaces the knife edge with a grating (similar to a very coarse diffraction grating) comprising fine parallel wires, an etching on a glass plate, a photograph negative or computer printed transparency.

[10] The Dall null test uses a plano-convex lens placed a short distance in front of the pinhole.