This tool allows a user to search the Splatalogue,7.1 a database containing millions of spectral lines. A subset of the Splatalogue is packaged with the Observing Tool, which should be sufficient to select some of the most commonly targeted transitions, even when working offline. If network connectivity is available, access to the complete catalogue becomes possible using the online search functionality of the tool.
The tool is available either when creating a Spectral Setup in a Science Goal (see Section 5.3.3) or an Instrument Setup in a Scheduling Block (Section 6.2.4.2). In both cases, the selected transitions can be used to define observed frequencies as well as being available to overlay on the Visual Spectral Editor. In addition, a separate instance of the tool on the spectral-setup page of a Science Goal allows rest frequencies to be defined (Section 5.3.3.10).
When the transition selector dialogue is launched, the offline spectral line catalogue is displayed and every transition in the catalogue is available for selection. Specific transitions can be located by using the controls in the left-hand panel to set your search criteria. String matches to the transition name or description are possible, as is matching by frequency and ALMA instrument band by dragging the range sliders or using the frequency input boxes. The string matching is not standard: a bare piece of text will only match the beginning of a string, but if a wildcard ('*') is placed at the beginning, the text will match anywhere (and not just at the end). Note that the frequency slider refers to sky frequencies (these are calculated using the velocity of the first source in the Science Goal). Double-clicking on the column headings will cause the table to be sorted according to the values in that column (large triangle) and clicking a subsequent column will cause a secondary sort (small triangle). The triangles indicate the direction of the sort and subsequent single clicks will reverse this.
If a desired line cannot be found in the offline catalogue, it may exist in the more extensive online catalogue. The online catalogue can be queried by clicking the ``Find More...'' button; the query will use the species filter and the frequency limits only and wildcards are not allowed. The results of the query will be added to the list of selectable transitions, and will persist as part of the offline database for the rest of the observing tool session.
Having located a target transition, it can be copied into the table of selected transitions by selecting it and clicking the obvious button, or simply by double-clicking. When creating a spectral setup, only one line can be chosen at a time. However, when overlaying, multiple lines can be copied simultaneously into, and removed from, the table of selected transitions, by highlighting multiple rows. Closing the tool using the ``OK'' button will cause the selected frequencies to be entered into the OT.
A very important part of the Line Picker is its ability to only allow lines to be selected that can be observed together in a single tuning solution. This ``unobservable lines'' filter is enabled by default and calculates the range of frequencies for which a line could be entered to the currently defined list of spectral windows (even if they are in other basebands) and still result in a valid tuning. In overlaying mode, it instead restricts the list to those lines that could fit the current tuning solution. Note that this feature makes no allowance for the width of a spectral line so it is important to check that all spectral lines are properly covered by their spectral window.