Ronchi test
method of determining the surface shape of a mirror used in telescopes and other optical devices
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.
Description
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. [1]. 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 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.
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