|
For historical reasons, the velocity convention used to stop the periodic
shift due to Earth rotation (see Sect. ) has always been the
optical convention at the IRAM-30m. The systemic velocity given by the
observer is then interpreted as a radio velocity by MIRA and MRTCAL
to compute the doppler factor from the observatory to the source rest
frame. This is exactly the situation described in
Sect.
. Figure
shows the relative
error on the dilatation factor (left panel) and the frequency offset for a
line at 115 GHz (right panel).
The corresponding frequency offset is 3.0 kHz for a velocity of 28
,
and 1.0 MHz for a velocity of 885
. Hence the position of the tuned
line for both Galactic and extra-Galactic measurements is affected in a
measurable way. Moreover, the dilatation factor differs from 1.0 by a value
of the order of
for a velocity of 885
. This means that the
frequency of the lines that were not tuned will be wrong by 1/10th of
channel when they are located 10000 channels from the tuned
frequency. This is a 2nd order effect that is more difficult to measure
with today generation of receivers but it may become measurable with future
generations.
To fix this behavior, we first propose to modify the way the spectroscopic
section of CLASS data is filled by MRTCAL as follows
![]() |
(78) |
![]() |
(79) |
A more consistent solution would be that the 30m changes the formula it uses to stop the periodic shift due to Earth rotation and now uses the radio convention.