Influence of photon count on differential piston estimation accuracy. Differential piston RMSE versus number of photons for the SP25 turbulence configuration, with 13-frame averaging. Red dotted line represents the noise-free limit from 13-frame averaged SP25 in Fig. 5. Top axes show the corresponding source magnitude, assuming an ELT-like system with a wavefront sensor operating at 500 Hz. The lower top axis corresponds to 13-frame averaging under the uncorrelated regime, while the upper top axis corresponds to the effective integration time of ~1 s in real-life operation (see Sect. 4.3 for details).