Keywords: class-spectral-axis-v12

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## 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`.