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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, usually using high spectral-resolution correlator modes (FDM) although low spectral-resolution modes (TDM) are also available, either for wide spectral lines or continuum determination. A mixture of these can be observed in the same spectral setup (Figure 5.7). A line peak flux density and a line width must be entered into the expected source properties section of the Field Setup form.

Within each of the four available basebands, a single correlator mode can currently be used, and the bandwidth of this can either be observed as a single or multiple spectral windows. These can either be entered manually or via the Spectral Line Selection tool which will set the frequency automatically (see Section 7.3). Once a row in the spectral line table has been entered, it is possible to choose the bandwidth and spectral resolution of the spectral window and, for FDM, a meaningful name (e.g. spectral transition) must be entered. The single TDM spectral window is shown in bold font in order to distinguish it and, although its bandwidth is stated as 2000 MHz, its effective bandwidth is actually only 1875 MHz (the correlator outputs the full 2000 MHz though).

As Hanning smoothing is enabled by default in the ALMA correlator, the displayed frequency resolution will always be at least twice that of the channel spacing. Further smoothing is possible by averaging channels in the correlator. This has the advantage of reducing the data rate and volume and therefore making data transport and reduction much easier. Spectral averaging of the relevant spectral windows is done by selecting a power of two (up to a maximum of 16) from the drop-down list. This is done on a per spectral window basis and the displayed spectral resolution will be updated to reflect the choice of averaging.

Note that, because of the Hanning smoothing, averaging channels does not simply cause the spectral resolution to increase by the averaging factor. However, a rule of thumb is that as the averaging factor is increased, the spectral resolution tends towards the channel spacing multiplied by the averaging factor. It is not possible to average TDM spectral windows as these already have very low spectral resolution.

Multiple spectral windows per baseband (known as Multi-Region Mode) are created in the correlator by moving the individual 62.5 MHz-wide tunable filters within the confines of the baseband - this therefore only makes sense for modes with a bandwidth of 1 GHz or less. The splitting up of modes is done using the ``Fraction'' column in the spectral window table. This changes the modes available in the drop-down list by e.g. halving the bandwidths, and different fractions therefore allow spectral windows with a variety of bandwidths. Note that the spectral resolution (before spectral averaging) of each must be identical, or else they cannot be said to correspond to the same correlator mode. The sum of the fractions must (obviously) not exceed unity, although (less obviously) they do not have to add up unity i.e. it is not compulsory to use the full mode. Multi-region mode is not possible using TDM where it will always only be possible to select one maximum-bandwidth spectral window per baseband.

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 (by setting the opacity and aperture efficiency 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 (Chapter 9).

The Spectral Visual Editor is very useful as it provides a clear, graphical overview of the spectral setup including the spectral windows and their location within the sideband. At Band 9, the ``image'' versions of the user-defined ``signal'' spectral windows are also shown. Astronomical emission and atmospheric noise from the image spectral windows find their way into their signal equivalents, although the correlator is able to reject the astronomical component. The noise, however, will remain and therefore regions of low atmospheric transmission should also be avoided in the image spectral windows.


next up previous contents
Next: Single Continuum Up: Spectral Setup Previous: Spectral Setup   Contents
The ALMA OT Team, 2014 May 21