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.