Spectral diagnostics for hydrocarbon absorption and CR computation. Transparent green and cyan bands indicate the absorption spectral ranges used for CR calculations (125-130 nm for CH₄, 150153 nm for C₂H₂), while the magenta band represents the nonabsorbed region assumed to be free of hydrocarbon absorption. The orange band is the new spectral range proposed for absorption of CH₄ and the purple one is used to scale the color ratio in case of detector nonlinearity in the unabsorbed band (see Sect. 3.3). Upper panel: Simulated unabsorbed H₂ spectrum from TransPlanet in dark blue. Lower panel: Corresponding optical depth for CH₄, C₂H₂, and C₂H₆, showing the ranges used in CR calculations (from Benmahi et al. 2024a).