Signal of γ vs. α correlation as a function of channel k and image I (noted η_kΔy_I, with Δy_I fixed to 0 for I = 24) (blue). Residual of the fit of this signal with one η_k parameter per channel and one Δy_I per image (black). Images I = 2, 17, 18 were called w, u, v in the spectral studies of Sect. 2. Δy is calibrated by comparison with Δy_{w → u}. The inset shows a Gaussian fit of the residuals (1 point/channel/image) with a 5 × 10^-4 rms, yielding Δy_k = 0.05 μ or ⟨ Δy_k ⟩ = 0.006 μ. The green line indicate the effect of a 1 μ LED drift.