Ronchi test
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In optical testing a Ronchi test is a method of determining the figure of a mirror used in telescopes and other optical devices.
[edit] Description
Described in a 1922 published work by Italian physicist Vasco Ronchi, the Ronchi test uses simple equipment in the testing of optics. [1]. A "Ronchi tester" consists of:
- A light source
- A diffuser
- A diffraction grating
The light source is emitted through the diffraction grating, reflected by the mirror being tested, then passes through the diffraction grating again and observed by the person doing the test.
The result is a pattern of interference that reveals the shape of the mirror. The interference is compared to a mathematically generated diagram (usually done on a computer today) of what the pattern should look like for a given figure. Inputs to the program are line frequency of the diffraction grating, focal length and diameter of the mirror.
[edit] Applications
The Ronchi test is used in the testing of mirrors for Reflecting telescopes especially in the field of Amateur telescope making. It is much faster to set up than the standard Foucault test but, unlike the Foucault tester, the Ronchi test requires a piece of specialized equipment (the diffraction grating) and is more subjective. It offers a quick glimpse at the mirrors 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.
