A series of measurements with the tester, finding the radii of curvature of the zones along the optical axis of the mirror (Y-axis). These data are then reduced and graphed against an ideal parabolic curve. Other testing techniques A number of other tests are used which measure the mirror at the center of curvature. Some telescope makers use a variant of the Foucault test called a Ronchi test that replaces the knife edge with a grating (similar to a very coarse diffraction grating) comprising fine parallel wires, an etching on a glass plate, a photograph negative or computer printed transparency. Ronchi test patterns are matched to those of standard mirrors or generated by computer. Other variants of the Foucault test include the Gaviola or Caustic test which can measure mirrors of fast f/ratio more accurately than the Foucault test which is limited to about (λ/8) wavelength accuracy on small and medium-sized mirrors. The Caustic test is capable of measuring larger mirrors and achieving a (λ/20) wave peak to valley accuracy by using a testing stage which is adjusted from side to side so as to measure each zone of each side of the mirror from the center of its curvature. The Dall null test uses a plano-convex lens placed a short distance in front of the pinhole. With the correct positioning of the lens, a parabolic mirror appears flat under testing instead of doughnut-shaped so testing is much easier and zonal measurements are not needed. There are a number of interferometric tests which have been used including the Michelson-Twyman and the Michelson method, both published in 1918, the Lenouvel method and the Fizeau method. Interferometric testing has been made more affordable in recent years by affordable lasers, digital cameras (such as webcams), and computers, but remains primarily an industrial methodology. See also Schlieren photography Airy disk Amateur telescope making Angular resolution (see Angular resolution#Explanation for discussion of the Rayleigh criterion) Diffraction-limited system Huygens–Fresnel principle#Single slit diffraction Fabrication and testing of optical components Null corrector Strehl ratio References Further reading L. Foucault, "Description des procedees employes pour reconnaitre la configuration des surfaces optiques," Comptes rendus hebdomadaires des séances de l'Académie des Sciences, Paris, vol. 47, pages 958-959 (1858). L. Foucault, "Mémoire sur la construction des télescopes en verre argenté," Annales de l'Observatoire impériale de Paris, vol. 5, pages 197-237 (1859). Amateur astronomy Measuring instruments Mirrors Optical devices Telescopes