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The previous equations assume that the systemic velocity is given in the
same frame as the frequency channel spacing. However, at the 30m the
systemic velocity is usually given in the LSR frame (as requested by the
user) and the channel spacing and the doppler factor are given in the
observatory frame.
On one hand, the rest frequency scale is correctly computed by CLASS
because the channel spacing and the doppler factor are given in the same
frame. On the other hand, the velocity scale is only approximated because
the source systemic velocity is given in another frame, most often the LSR
frame.
The end-user (astronomer) is interested with the velocity scale in the LSR
frame
 |
(39) |
But he actually gets
 |
(40) |
The relative error can be expressed as
 |
(41) |
We yield to first order in
 |
(42) |
If we tolerate an error of 1/10th of a channel on the velocity, this
implies that the velocity scale is correct for about 1000 channels around
the systemic velocity. This translates into
at 1
for a
spectral resolution of 195
. This means that the interpretation of the
velocity scale of Galactic observations is correct. On the other hand, this
could be a problem for extra-galactic observations at extremely high
spectral resolution, assuming that the observers do not first downsample
their spectra to increase the signal-to-noise ratio per channel...
The easiest way out of the problem would be to write the parameters of the
header in the LSR frame in the calibration software that writes the CLASS
observation with
 |
(43) |
In other words, we trick CLASS into thinking that the measurement frame
is LSR, while the true measurement frame was the observatory.
Next: The MODIFY FREQUENCY command
Up: CLASS implementation
Previous: The header parameters
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Gildas manager
2023-06-01