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For historical reasons, the velocity convention used to stop the periodic
shift due to Earth rotation (see Sect. 3.2) is the optical
convention at the IRAM-30m. Let's assume that the systemic velocity of the
source is given in the radio convention as it is the natural convention for
radio-observatories (see Sect. 1.2.3). The tuned frequency
in the observatory frame is thus
 |
(101) |
while it should be
 |
(102) |
The output spectrum will appear shifted in the observatory frame by
 |
(103) |
And the signal frequency in the rest frame is shifted by
 |
(104) |
At 115 GHz, this gives a shift of 3.2 kHz for a velocity of 50
and
1.3 MHz for a velocity of 1000
. Hence both Galactic and
extra-Galactic measurements are affected in a measurable way. Nevertheless,
this can be measured only by comparison of the lines measured at the
IRAM-30m with other radio-observatories.
We recommend that the 30m changes the formula it uses to stop the periodic
shift due to Earth rotation and now use the radio convention.
Next: Systemic velocity in different
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Gildas manager
2015-03-19