Same simulation as in Fig. 4 but with a fourth mirror (if n is the mirror numbers: n(n − 1)/2 = 6 fringes and  triplets). Top: all the mirrors are in phase. Middle: the three mirrors of Fig. 4 are still in phase, but a 3 micron piston error has been added on the fourth mirror M₄. We wrote numbers in red, which are close the intersection of each triplet of fringes. The number 1 corresponds to the triplet of fringes without piston error of Fig. 4. Bottom: position at scale of the four mirrors.