In Cycle 2, multi-region correlator modes are offered for spectral line observations. This means that you can specify up to 4 spectral windows per baseband, yielding a maximum total of 16 spectral windows in a single spectral setup containing 4 basebands. While different basebands can use different correlator modes (i.e. the bandwidth and spectral resolution chosen for one baseband is independent of the choice made for the other basebands), all spectral windows within a baseband must use the same correlator mode. Because only a fixed number of channels are available for each baseband they must be divided between the spectral windows defined, implying that at a given resolution the bandwidth available for each of two spectral windows will be half that available for a single spectral window.
Selecting Spectral Line as Spectral Type will present you with the spectral window tables, as well as buttons to Select Lines to Observe, Add or Delete transitions. These actions must be performed individually for each baseband. Adding a spectral window manually is straightforward: press the Add button and simply type the frequency in the appropriate field in the table. In contrast to the Continuum setup case you can specify the frequency either in the sky or the rest frame, and the OT will adjust the sky frequency taking into account the velocity of different sources. The sky frequency displayed in the spectral window table refers to the source that is highlighted in the Targets table.
Alternatively, you can Select Lines to Observe in baseband... from a catalogue. Pressing this button will bring up the Spectral Line Selector tool (see Fig. 3.4). Here, you can search an extensive database (the Splatalogue3.1) for popular transitions based on species, ALMA band, Sky frequency, maximum upper-state energy or molecules detected in a variety of astronomical sources. If you cannot find the transition you want in this offline database, try the Find More... button: you will then be able to search online from a more extensive database. Once you have identified the transition you are interested in, select it and press the Add to Selected Transitions button. You may add up to 4 transitions, which will each be placed in a separate spectral window within the baseband. The Hide unobservable lines option, ticked by default, allows you to display only transitions that are simultaneously observable with the transition(s) you have already selected. When you press Ok, this will add one or more spectral windows to the baseband table with a centre frequency equal to the transition frequency. You can verify all the transitions both in the baseband tables and visually in the spectral editor (see Fig. 3.5).
The OT will try to configure a spectral setup based on your input. In the spectral viewer, the sidebands are shaded in yellow and the spectral windows selected are indicated by the blue vertical (line frequency) and horizontal (bandwidth) lines. The latter are labelled according to the transition they refer to. If the OT cannot configure a valid spectral setup based on your input transitions, you will see a red error message in the Spectral Setup Errors box and in the spectral viewer the sidebands will be faded to grey rather than appearing in yellow. In order for your proposal to pass validation, you must select a valid spectral setup, where all spectral windows for all source velocities can fit within a valid sideband configuration. Note that for Bands 3, 4, 6, 7 and 8 it is not possible to place 1 baseband in one sideband and 3 in the other, however 2 and 2 is acceptable, as is 4 spectral windows in one sideband. Band 9 has no such restrictions.
You can specify the bandwidth and spectral resolution (Hanning smoothed, corresponding to twice the channel spacing) of a spectral window by double-clicking on the Bandwidth, Resolution field in the spectral window tables, and choosing one of the options from the drop-down menu. If you have selected multiple spectral windows in a baseband, the fraction of the baseband assigned to each spectral window must be set in such a way that the sum of all fractions does not exceed 1. This will update the available choices for the Bandwidth, Resolution field to reflect the lower number of channels available. If you increase the bandwidth you should ensure that the OT can still configure a valid spectral setup. It is possible to change the centre frequency of spectral windows from the transition frequency in order to fit the spectral windows within the sideband, but please ensure that your lines are still covered by the spectral windows. Of course, if you cannot define a valid spectral setup for all the spectral windows you wish to observe, you may also put them into separate Science Goals.
In Cycle 2, it is possible to spectrally average each spectral window in order to lower the data rate and the size of the output data file. This is very easily done by double-clicking on the Spec Avg field in the spectral window tables and selecting one of the options from the drop-down menu. The Bandwidth, Resolution field belonging to that spectral window will immediately be updated to reflect the new effective resolution. Note that this is not a simple multiplication by the spectral averaging factor; for example a spectral averaging of 2 will yield a resolution only slightly inferior to the non-averaged case. The spectral averaging can be set independently for each spectral window. The effect that this has on the data rate can be seen on the proposal or science goal summary sheets (see Step 3.10).
Finally, you should carefully check the Representative Frequency displayed below the baseband tables and modify it if necessary (see previous section). For spectral line observations, the representative frequency should normally correspond to your transition of greatest interest, or if you are interested in several lines the one that has the poorest atmospheric transmission.