The radio velocity convention
Keywords: class-spectral-axis-v12

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The radio velocity convention

If the radial velocity is negligeable compared to the light velocity, we obtain to first order

| (68)

In this case, we can establish a linear velocity scale (see Sect. )

| (69)

This linear velocity scale is only a first order approximation, which is called the radio velocity convention. Indeed, it is well adapted to the radio spectrometer because 1) their natural output is a spectral axis regularly spaced in frequency and 2) the radio velocity convention gives a linear relation between the velocity and the frequency scale. This is why this convention is the default in `CLASS`.