## The optical velocity convention

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.