The IRAM-30m case as of 2019-07-06

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The IRAM-30m case as of 2019-07-06

Figure: Left: Relative error on the dilatation factor. Right: Frequency offset for a line at 115 GHz. The red lines correspond to typical LSR velocities for Galactic/extra-Galactic sources, respectively.

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 10 000 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)
The inconvenient of this solution is that it makes , , and dependent on time because the correction contains the Doppler effect from the observatory to the LSR frame.

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.