Spectral-line observing with VLBI is significantly different. At the moment it is only possible to carry out line observing in Band 1, 3, 6 and 7. For Band 1 and Band 3, baseband 1 needs to be in VLBI mode, i.e.,its bandwidth must be 1875 MHz, and only one spectral window is allowed there. For Band 6 and Band 7 at least one of the basebands must be in VLBI mode (but not necessarily baseband 1) and only one spw is allowed there. The other three basebands can be used to define additional spectral windows that will produce ALMA-only data. Their frequencies can be freely chosen, so long as a valid tuning solution can be found.
Another aspect of spectral-line VLBI that is non-standard is that the user must enter an observing date for the observation using the time-constraint interface (single visit). This is needed in order for the OT to confirm that the full width of the line will fall into the central 234 MHz of spectral window on the expected date, given the source's velocity relative to the telescopes. For this reason it is also necessary to define the expected line width in Expected Source Properties. If the line cannot be observed then there will be a validation error.
The ALMA OT Team, 2024 May 21