An example of the difference in models between a continuum source (red) and the CMB (green) as the background. The top plot shows the 1₁₀ − 1₁₁ line and the bottom shows the 2₁₁ − 2₁₂ line both with the continuum level set to zero in the plot. The residuals are shown offset above the spectra, with the dashed line indicating the zero-residual level. The grey shaded regions show the 1σ error bars on each pixel. The model with the CMB as the only background is able to reproduce the absorption line, while the model with the H ii region in the background cannot account for the depth of the 2₁₁ − 2₁₂ line. The reduced χ²/n for the models are 14.1 (red) and 2.8 (green), evaluated only over the pixels where the model is greater than the local rms.