Data obtained from the differential output of a double Bracewell nuller, showing the recorded counts as a function of wavelength and time. Top: signal of an Earth twin at 10 pc as recorded by an ideal instrument. Middle: total signal of all astrophysical sources, including the star, the local zodi, and the exozodi, as recorded by an ideal instrument. The counts are almost two orders of magnitude higher than for the pure planet signal and enter the data as white noise, predominantly at the low and the high wavelengths. Bottom: total signal of all astrophysical sources as recorded by a perturbed instrument. We have assumed an rms amplitude perturbation of 0.1%, an rms phase perturbation of 1.5 nm, and an rms polarization angle perturbation of 0.001 rad. The counts are again almost an order of magnitude higher and introduce strong spectral—but also temporal—correlations into the data, as apparent by the vertical stripes.