Experimental results on piston detection by using the classical Ronchi test.
We present some experimental results of piston detection by using a classical Ronchi test in a segmented spherical surface. We compare the Ronchi fringes frequency of both segments (reference and under test). When the piston term is present at the segment under test, it is moved iteratively until we obtain a minimum piston error. When the surface is almost cophased, both segments have the same fringe frequency. The fringes of each segment are compared by correlating the intensity versus position in each segment. The Ronchi test is used at the same time as Shack interferometry; this second method is used as a reference to avoid segments tilts. With the Ronchi test we have detected experimentally a piston of the order of 45 nm (lambda/14 with lambda=632.8 nm). This result was compared with images of simulated Ronchigrams.