optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...

testing a Ronchi test is a method of determining the surface shape ( figure) of a mirror used in

telescopes A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obser ...

and other

devices.

Description

In 1923

Italian Italian(s) may refer to: * Anything of, from, or related to the people of Italy over the centuries ** Italians, an ethnic group or simply a citizen of the Italian Republic or Italian Kingdom ** Italian language, a Romance language *** Regional Ita ...

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate ca ...

Vasco Ronchi Vasco Ronchi (; December 19, 1897 – October 31, 1988) was an Italian physicist known for his work in optics. He was born on 19 December 1897 in Florence, Italy. Along with Enrico Fermi, he was a student of Luigi Puccianti. He studied at the Fa ...

published a description of the

eponymous An eponym is a person, a place, or a thing after whom or which someone or something is, or is believed to be, named. The adjectives which are derived from the word eponym include ''eponymous'' and ''eponymic''. Usage of the word The term ''epon ...

Ronchi test, which is a variation of the

Foucault knife-edge test 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, inexpen ...

and which uses simple equipment to test the quality of optics, especially

concave mirror A curved mirror is a mirror with a curved reflecting surface. The surface may be either ''convex'' (bulging outward) or ''concave'' (recessed inward). Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are ...

s.Masud Mansuripur (July 1997) "The Ronchi test," ''Optics & Photonics News'', vol. 8, pages 42-46. Available on-line at: http://www.mmresearch.com/articles/article1/index.htm .

A "Ronchi tester" consists of: *A light source *A Diffuser (optics), diffuser *A Ronchi grating A Ronchi grating consists of alternate dark and clear stripes. One design is a small frame with several evenly spaced fine wires attached. Light is emitted through the Ronchi grating (or a single slit), reflected by the mirror being tested, then passes through the Ronchi grating again and is observed by the person doing the test. The observer's eye is placed close to the centre of curvature of the mirror under test looking at the mirror through the grating. The Ronchi grating is a short distance (less than 2 cm) closer to the mirror. The observer sees the mirror covered in a pattern of stripes that reveal the shape of the mirror. The pattern is compared to a mathematically generated diagram (usually done on a computer today) of what it should look like for a given figure. Inputs to the program are line frequency of the Ronchi grating, focal length and diameter of the mirror, and the figure required. If the mirror is spherical, the pattern consists of straight lines.

Applications

The Ronchi test is used in the testing of mirrors for

reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...

s especially in the field of

amateur telescope making ''Amateur Telescope Making'' (''ATM'') is a series of three books edited by Albert G. Ingalls between 1926 and 1953 while he was an associate editor at ''Scientific American''. The books cover various aspects of telescope construction and obser ...

. It is much faster to set up than the standard

. The Ronchi test differs from the knife-edge test, requiring a specialized target (the Ronchi grating, which amounts to a periodic series of knife edges) and being more difficult to interpret. This procedure offers a quick evaluation of the mirror's shape and condition. It readily identifies a 'turned edge' (rolled down outer diameter of the mirror), a common fault that can develop in

objective Objective may refer to: * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pronoun that is used as a grammatical object * Objective Productions, a Brit ...

mirror making. The figure quality of a convex lens may be visually tested using a similar principle. The grating is moved around the focal point of the lens while viewing the virtual image through the opposite side. Distortions in the lens surface figure then appear as asymmetries in the periodic grating image.

Footnotes

{{Reflist

References

Scienceworld - Wolfram.com - Ronchi test
Optics Telescopes