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Interpretation 1: At fixed
Let's assume that the same gas cell in the source emits two lines at
different frequencies. We are at fixed
and thus
because we
consider the same gas cell. The frequency axes in the rest and observatory
frames are thus given by
 |
(4) |
This means that the frequency separation between the two lines is different
in the rest and observatory frames. The modeller has easily access to the
rest frame frequencies of the line. It is thus important to display the
spectrum frequency axis in the rest frequency axis. This can be achieved
only for one velocity (The reasoning is here done at fixed
), which
is by default assumed to be the systemic velocity of the source in the
observatory frame, i.e., the mean velocity of the source gas in the
observatory frame, written
.
As a convention, CLASS assumes that the tuned rest frequency and its
corresponding observatory frequency at the source systemic velocity are
associated to the common reference channel
. CLASS then displays the
frequency axis in the rest frequency associated to the source systemic
velocity through
 |
(5) |
The plotted spectrum thus correctly displays the line at rest frequency
positions, i.e., the brightnesses of the gas whose velocity is equal to
the systemic velocity in the observatory frame.
Next: Interpretation 2: At fixed
Up: Introduction
Previous: The Doppler effect
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Gildas manager
2023-06-01