Classical Ronchi test for piston detection

We propose a method for piston errors detection in a segmented surface by means of the classical ronchi test. This consists in comparing the ronchigram fringes frequency of a reference and piston segment. The comparison is developed for the correlation method of the intensity vs pixels curves of the reference and piston segment. The presence of the piston term in a ronchigram is assured experimentally for the shack interferometer, it is by observing the coincidence rings centered type fizeau in each segment. The proposed method is applied to a segmented spherical surface with a two segment mirrors, and resolutions of piston g63 ηm are experimentally obtained.