Ronchi test
Encyclopedia
In optical
testing a Ronchi test is a method of determining the surface shape (figure
) of a mirror used in telescopes
and other optical
devices.
physicist
Vasco Ronchi
published a description of the so-called 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. http://www.atm-workshop.com/ronchi-test.html. A "Ronchi tester" consists of:
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 2cm) 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.
s especially in the field of Amateur telescope making
. 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. It offers a quick glimpse at the mirror's shape and condition, and can quickly identify a 'turned edge' (rolled down outer diameter of the mirror), a common fault that can develop in objective
mirror making.
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
testing a Ronchi test is a method of determining the surface shape (figure
Figuring
Figuring is the process of final polishing of an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.-Types of figuring:...
) of a mirror used in telescopes
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
and other optical
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
devices.
Description
In 1923 ItalianItalian people
The Italian people are an ethnic group that share a common Italian culture, ancestry and speak the Italian language as a mother tongue. Within Italy, Italians are defined by citizenship, regardless of ancestry or country of residence , and are distinguished from people...
physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
Vasco Ronchi
Vasco Ronchi
Vasco Ronchi 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...
published a description of the so-called Ronchi test, which is a variation of the Foucault knife-edge test
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 . It is commonly used by amateur telescope makers for figuring small astronomical mirrors...
and which uses simple equipment to test the quality of optics, especially concave mirrors. http://www.atm-workshop.com/ronchi-test.html. A "Ronchi tester" consists of:
- A light source
- A 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 2cm) 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 telescopeReflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
s especially in the field of Amateur telescope making
Amateur telescope making
Amateur telescope making is the activity of building telescopes as a hobby, as opposed to being a paid professional. Amateur telescope makers build their instruments for personal enjoyment of a technical challenge, as a way to obtain an inexpensive or personally customized telescope, or as a...
. It is much faster to set up than the standard Foucault knife-edge test
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 . It is commonly used by amateur telescope makers for figuring small astronomical mirrors...
but, unlike the Foucault tester, the Ronchi test requires a piece of specialized equipment (the Ronchi grating) and is more subjective. It offers a quick glimpse at the mirror's shape and condition, and can quickly identify a 'turned edge' (rolled down outer diameter of the mirror), a common fault that can develop in objective
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...
mirror making.