Normalised spectrum of the HFS splitting. The panels present the sub-mm (top) and FIR (bottom) transitions of CH connecting the ²Π_1/2, N = 2, J = 3/2 and ²Π_3/2, N = 1, J = 3/2 rotational levels to the ground state. The line intensities of the HFS triplets are computed assuming optically thin conditions at LTE for a Doppler line width at a gas temperature of 50 K. The velocity scale is displayed relative to the strongest HFS component.