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The radio velocity convention

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

\begin{displaymath}
\ensuremath{f_\ensuremath{\mathrm{}}^{\ensuremath{\mathrm{o...
...}}}^{\ensuremath{\mathrm{obs}}}}}{\ensuremath{c}} \right) }.
\end{displaymath} (68)

In this case, we can establish a linear velocity scale (see Sect. [*])
\begin{displaymath}
\ensuremath{v_{\ensuremath{\mathrm{}}}^{\ensuremath{\mathrm...
...{f_\ensuremath{\mathrm{tuned}}^{\ensuremath{\mathrm{rest}}}}}.
\end{displaymath} (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.



Gildas manager 2023-06-01