Interpretation 1: At fixed

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Interpretation 1: At fixed

Let's assume that the same gas cell in the source emits two lines at different frequencies. We are at fixed and thus because we consider the same gas cell. The frequency axes in the rest and observatory frames are thus given by

(4)
This means that the frequency separation between the two lines is different in the rest and observatory frames. The modeller has easily access to the rest frame frequencies of the line. It is thus important to display the spectrum frequency axis in the rest frequency axis. This can be achieved only for one velocity (The reasoning is here done at fixed ), which is by default assumed to be the systemic velocity of the source in the observatory frame, i.e., the mean velocity of the source gas in the observatory frame, written .

As a convention, CLASS assumes that the tuned rest frequency and its corresponding observatory frequency at the source systemic velocity are associated to the common reference channel . CLASS then displays the frequency axis in the rest frequency associated to the source systemic velocity through

(5)
The plotted spectrum thus correctly displays the line at rest frequency positions, i.e., the brightnesses of the gas whose velocity is equal to the systemic velocity in the observatory frame.