Spectral Line

Four panels for each baseband will be visible and up to four spectral windows (spw) can be added to each. Adding a spectral window is done either by selecting spectral lines from the spectral line catalogue or by adding them manually. One of the spectral windows should be selected as that which will set the Representative Frequency - this is known as the Representative Window. This defaults to the centre of the spectral window, but can be changed at the bottom of the page. This sets the size of the antenna beamsize drawn in the Visual Spatial Editor and is also used to work out the time estimate for the Science Goal.

For each baseband, the following buttons are available:

Figure 5.1: The spectral window table for a single baseband. Two spectral windows have been defined, each of which take up half of the available correlator capacity. Note that each has the same spectral resolution (this must be the case) and that the bandwidths are proportional to the assigned fraction. Further note that if there were different spectral averaging factors per spectral window, the spectral resolutions would not appear to be the same, although the bandwidths would still indicate the chosen fraction.
Image SpectralWindows

The spectral window table (Fig. 5.1) has the following columns. Most accept input by being double-clicked.

The value of the Representative Frequency is displayed below the table of spectral windows. Its value will default to the centre of the chosen spectral window, but can be changed to another frequency within that spectral window if the transition of greatest interest is not at the centre. For spectral line setups this is defined in the rest frame of the source. This is so that sources with different velocities end up with the appropriate Representative Frequency in the observed frame.

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. Spectral windows can be added to the other basebands and their properties can be freely edited - VLBI data will only result from the VLBI spw though. It is also necessary to enter a date for the observation in the time-constraint interface (Control & Performance) in order for the OT to check that the line will be observable.

The ALMA OT Team, 2024 Mar 01