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Spectral Line

Figure 5.7: Example of a Science Goal Spectral Setup Form. In this setup, four spectral windows have been defined. The first baseband contains an observation of CO in a narrow, high-resolution, spectral window and was created using the Spectral Line Picker. The other three basebands all contain a single, low-resolution, spectral window in order to determine the source continuum. They were entered manually and are automatically described as such by the OT. The CO spectral window has been selected to set the Representative Frequency.
Image SpectralSetup

As said above, the spectral line interface gives maximum flexibility in what can be defined. It will in fact generally be used to observe spectral lines, although the ability to mix FDM and TDM windows means that a mixture of high-resolution and low-resolution spectral work and continuum determination can be observed in the same setup (Figure 5.7).

Within each of the four available basebands, a single spectral window can be specified. Each window can either be added manually and its central frequency entered by hand, in the sky or rest frame, or else the Spectral Line Selection Tool (``Select Lines to Observe...'') can be used to search the vast Splatalogue spectral line database (see Section 7.3) and enter the frequency for you (along with a transition name). Once the frequency has been defined, a correlator mode (bandwidth and resolution) must be chosen from a drop-down menu that is activated by double-clicking - the single mode that is displayed in bold text is a TDM. Note that Hanning smoothing is disabled by default in the ALMA correlator and so the displayed frequency resolution is actually twice that of the channel spacing.

If the entered frequencies are not able to be observed simultaneously e.g. they do not all lie within the receiver sidebands, an error message will appear in red below the spectral window table and the proposal will also not validate. Using the Visual Spectral Editor is an excellent way of diagnosing spectral setup errors as it displays the lines and their bandwidths as well as the ALMA bands and sidebands. When choosing lines with the Selection Tool, its unobservable line filter will ensure that lines cannot be picked if they can't be observed simultaneously with lines that have already been defined.

The final column of the spectral window table selects which is to be used to set the Representative Frequency. This is a very important parameter as it, in conjunction with the sensitivity entered in the Control and Performance page, sets the total observation time as it determines the opacity that is used in the sensitivity calculation. It also sets the antenna beamsize in the Visual Spatial Editor. If the transition you are most interested in does not fall in the centre of the chosen spectral window, the Representative Frequency can be changed to the appropriate value, with the restriction that the edited frequency must lie within the spectral window. The Representative Frequency interface is also shown in Figure 5.7.

At the bottom of the page is a list of all the sources that have been defined in the Field Setup page and which are listed together with their velocity specifications and Representative Frequencies. The latter are given in the sky (observed) frame as this is relevant quantity for determining the observing time for each source. Clicking on a source will use that source's velocity for the rest-to-observed frame and velocity conversions in both the spectral window table and the Spectral Visual Editor.


next up previous contents
Next: Single Continuum Up: Spectral Setup Previous: An introduction to the   Contents
The ALMA OT Team, 2013 Feb 27