Optical design of the OHP prototype metrology simulated with the Zemax software. On the left, a scale schematic view similar to that shown in Fig. 3 illustrates the plane including the 9   m baseline and the C point. In the middle, a zoom of the field mirror is shown: the ray tracing is for a 1   mm offcenter metrology mirror (horizontal displacement). On the right, the two images give the results of simulations performed, respectively, without and with piston errors (on the N mirror). In the two cases, the fringes of the three baselines at OHP (5   m, 9   m, 10.5   m) are shown.