(a) Linearity of the phase diversity on the estimation of a piston aberration on the central segment (range of piston values from 0 to 200 nm), in the direct case (no coronagraph). (b) Linearity of the phase diversity on the estimation of a tip aberration on the central segment (range of values from 0 to 200 nm pv). In both cases, the theory curves correspond to y = x. In the piston case (a) the curve can be approximated with the linear function y = 0.932(±0.006)× − 4.02(±0.97) and in the tip case (b) it can be approximated with the linear function y = 0.857(±0.009)× − 0.98(±1.2). These plots show a possible uncertainty in the calibration of Iris-AO.