Optical spectrometers (e.g., grates) naturally delivers a spectral axis
regularly spaced in wavelength. Eq.
can then
be rewritten as
(70)
The first order approximation gives
(71)
In frequency this gives
(72)
Following the same path as in Sect.
but now
in wavelenght, it is easy to show that we can establish a linear velocity
scale proportional to the wavelength axis
(73)
This velocity scale is called the optical velocity convention. The relation
between this velocity scale and the frequency spectral axis is
non-linear. This is why it is not used in CLASS.