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Optical spectrometers (e.g., grates) naturally delivers a spectral axis
regularly spaced in wavelength. Eq. 4 can then be
rewritten as
 |
(8) |
The first order approximation gives
 |
(9) |
Following the same path as in Sect. 1.3.2 but now
in wavelenght, it is easy to show that we can establish a linear velocity
scale proportional to the wavelength axis
 |
(10) |
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.
Gildas manager
2015-03-01