Example of the OD ²Π_3∕2 ground-state(J = 5∕2−3∕2) absorption towards the quiescent source G23.21-0.3. The spectra are normalised to the (single side-band) continuum level, and thus show the line to continuum ratio. The rest frequency is set to the strongest hyperfine transition, F = 7/2⁻–5/2⁺, at 1391494.70 MHz. The blue dotted line marks the systemic LSR velocity of the source, and the grey dotted line shows the value of1. The blue line shows a single component Gaussian fit to the spectrum. The positions and the relative strength of the hyperfine components are shown in green.