Differential intensity response, R ₄ − R ₃, of a double Bracewell nuller in the case of an ideal (left) and a perturbed (right) instrument at 10 μm at a given instant in time. The impact of the perturbations is highly exaggerated (rms(δA _k ) = 10%, rms(δϕ _k ) ≈ 0.28 rad, rms(δθ _k ) = 0.4 rad) here for illustrative purposes. The dashed line corresponds to the path that a planet traces out as the interferometer array rotates around its line of sight to the star.