Power spectrum of surface-brightness fluctuations (red points) fit with a power law taking covariance into account. The dashed black line and cyan envelope denote the 68% confidence region obtained using MCMC. The green and blue data points show the analytical expectation for the Poisson noise and the power spectrum recovered from a simulation including Poisson noise and unresolved point sources. Right: characteristic amplitude of the fluctuations (red) compared to the simulated Poisson noise (blue).