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Foucault test setup to measure a mirror. The Foucault knife-edge test is an optical test to accurately measure the shape of concave curved mirrors. It is commonly used by amateur telescope makers for figuring primary mirrors in reflecting telescopes. It uses a relatively simple, inexpensive apparatus compared to other testing techniques.
Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes ...
Geometrical optics. Geometrical optics, or ray optics, is a model of optics that describes light propagation in terms of rays. The ray in geometrical optics is an abstraction useful for approximating the paths along which light propagates under certain circumstances. The simplifying assumptions of geometrical optics include that light rays:
R = radius of curvature, R > 0 for concave, valid in the paraxial approximation θ is the mirror angle of incidence in the horizontal plane. Thin lens f = focal length of lens where f > 0 for convex/positive (converging) lens.
The lens is moved until a sharp image is formed on the screen. In this case 1 / u is negligible, and the focal length is then given by . Determining the focal length of a concave lens is somewhat more difficult. The focal length of such a lens is defined as the point at which the spreading beams of light meet when they are extended ...
This image is located at the conjugate distance to the lens according to the thin lens equation: = + where is the focal length of the lens, is the distance from the object to the lens and is the distance from the image of the object to the lens. A knife edge at the point source-image location is positioned as to partially block some light from ...
Schlieren photography is a process for photographing fluid flow. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects. The process works by allowing normally unobservable changes in a fluid's refractive index to be seen, [ 1 ...
In 1923 Italian physicist Vasco Ronchi published a description of the eponymous Ronchi test, [1] [2] which is a variation of the Foucault knife-edge test [3] [4] and which uses simple equipment to test the quality of optics, especially concave mirrors. [5] [6]. A "Ronchi tester" consists of: A light source; A diffuser; A Ronchi grating