Ronchi test

The Ronchi test is an efficient way of checking the shape of a telescope mirror. Conceived by the Italian optician Vasco Ronchi (1897–1988) in 1923, it is based on the Foucault knife-edge test but instead of a straight edge it uses a small diffraction grating with about 20 to 40 lines per mm. The grating is put in front of the eye after removing the eyepiece and pointing at a star. Any curvature of the lines indicates deviations from a spherical mirror.