Impact of the target system configuration on the ratio of fundamental noise to instrumental noise on a grid of amplitude and phase perturbations for the shortest-wavelength bin at λ_b = 4.1 µm with width ∆λ_b = 0.2 µm. The system parameters assumed for the M-star target are given in the text. The contours indicate where the fundamental noise dominates the systematic noise by a factor of one, five, or ten, respectively. The cross indicates the 4 µm reference level specified by Table A.1.