Average of nearly 5× 10⁴ (8.3 min) FSP measurements acquired with the modulator package disconnected. Only a fraction of the sensor area is shown. The contrast of the averaged, non-flat-fielded Stokes I image (⟨ I ⟩) is practically dominated by the etalons cavity errors. The difference of the artifact introduced by the bias term of Eq. (2), is evident in the Stokes Q images that were accumulated and then normalized (⟨ Q ⟩/⟨ I ⟩) and vice versa (⟨ Q/I ⟩).