Foucault knife-edge test

The so-called "Foucault knife-edge test" allows the worker to tell if the mirror is perfectly spherical or has non-spherical deviation in its figure.

The equipment most amateurs use to test the shape of the mirrors, a Foucault knife-edge test, is, like the tools used to create the surface, simple to fabricate.

Direct surface surveying (no intervening optics, for example Foucault knife-edge test, Ronchi test, Caustic test)

Testing of the figure is usually done by a Foucault knife-edge test or Ronchi test in amateur telescope making and with very sophisticated null testers on research telescope optics.

It is much faster to set up than the standard Foucault knife-edge test but, unlike the Foucault tester, the Ronchi test requires a piece of specialized equipment (the Ronchi grating) and is more subjective.

In 1923 Italian physicist Vasco Ronchi published a description of the eponymous Ronchi test, which is a variation of the Foucault knife-edge test and which uses simple equipment to test the quality of optics, especially concave mirrors.

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).