MAROON-X transmission spectrum of WASP-189 b for the Ca⁺ infrared triplet stacked in the planetary rest frame for both nights combined. The rest frame transition wavelengths are marked with dashed vertical lines in grey. The data is shown in grey. The binned data (×8) is shown in black. Upper panels: models for the leading (blue) and trailing (red) terminator computed using the T-p profiles in Fig. 4 assuming chemical equilibrium and solar metallicity. The models are sampled at the resolution of the spectrograph (R ~ 85 000, υ = 3.52 km s⁻¹), and additionally broadened to match the planetary rotation (υ_rot sin i = 3.04 km s⁻¹, Prinoth et al. 2023) and exposure smearing (υ_smear,red ≈ 0.14 km s⁻¹). Additionally, the models were continuum-normalised by computing their continuum separately and subtracting it. Middle panels: the data has been corrected for tellurics and the residuals of the Rossiter–McLaughlin effect using the parameters in Table 3. The best-fit Gaussians are shown in red. The templates for Ca⁺ at 4000 K and Fe at 3000 K are shown to compare the line position. Bottom panels: residuals after removing the best fit.