Ronchi test

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.


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 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.


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 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.


  1. V. Ronchi (1923) "Le frange di combinazioni nello studio delle superficie e dei sistemi ottici" [Combination fringes in the study of surfaces and optical systems], Rivista d'Ottica e Meccanica di precisione [Journal of Optics and Precision Mechanics], vol. 2, pages 9-35.
  2. Vasco Ronchi (1927) "Due nuovi metodi per lo studio delle superficie e dei sistemi ottici" (Two new methods for the study of surfaces and optical systems), Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Series 1, vol. 15, pages 1-50. [Note: This citation is somewhat misleading. Ronchi first published this article in 1923, but it was included in volume 15 of Annali della Scuola..., which wasn't issued until 1927.] Available on-line (in Italian): .
  3. L. Foucault (1858) "Description des procedes employes pour reconnaitre la configuration des surfaces optiques" (Description of the methods used to recognize the configuration of optical surfaces), Comptes rendus hebdomadaires des séances de l'Academie des Sciences, Paris, vol. 47, pages 958-959.
  4. L. Foucault (1859) "Mémoire sur la construction des télescopes en verre argenté" (Memoir on the construction of reflecting telescopes), Annales de l'Observatoire impériale de Paris, vol. 5, pages 197-237.
  5. J. A. Anderson and Russell W. Porter (1929) "Ronchi's method of optical testing," Astrophysical Journal, vol. 70, pages 175-181. Available on-line at: .
  6. Masud Mansuripur (July 1997) "The Ronchi test," Optics & Photonics News, vol. 8, pages 42-46. Available on-line at: .


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