Comparison between expected phase and estimated phase when the Iris-AO is flat except for the central segment on which a 225 nm piston is applied, in presence of the Lyot coronagraph: (a) theoretical phase map computed from the command sent to the Iris-AO, in radians, (b) phase map reconstructed by COFFEE, in radians, (c) vertical cross-sections of these two phases. Extra low-order aberrations are reconstructed, in addition to a underestimation of the local piston on the central segment.