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Spectral Scan

For projects which require more frequency coverage than a single tuning can provide, a short-cut to a multiple-tuning, continuous frequency coverage setup is provided by the Spectral Scan interface. Examples of such a project include the search for multiple CO lines for redshift measurement reasons, or a survey of multiple species in the dense line forest of a molecular cloud. Spectral scans can only be used for observations of a single offset pointing per source and the ACA is not allowed.

A spectral scan is defined by entering a start and stop frequency, in the observed/sky frame only, as well as the correlator mode that should be used. It is only possible to select spectral windows with a width greater than or equal to 500 MHz. Using these parameters the OT will then attempt to cover the requested frequency range using a maximum of five tunings. This maximum results in it not always being possible to cover the requested range and additional Science Goals should perhaps be created if significantly more coverage is required. The OT will issue a warning is this is the case. Also, the algorithm used to calculate the tunings usually assumes that all spectral windows of a tuning must be used - this leads to the actual range being observed usually differing from that requested. Spectral averaging is available, but will be the same for each spectral window of each tuning.

In more detail, spectral scans involving only a single sideband (either because the 500 or 1000 MHz-wide spectral windows are being used, or because the requested frequency coverage is relatively small) are calculated by placing the lowest frequency of tuning $n+1$ directly next to the highest frequency of tuning $n$. No overlap is assumed, either between the spectral windows of a tuning, or between tunings. This ensures maximum frequency coverage and should still ensure a seemless join between the tunings as, when placing the tunings, the OT assumes the effective bandwidth for the FDM spectral windows i.e. edge channels are already flagged. For TDM, the OT also uses the effective bandwidth although the edge channels are not flagged in the output data.

A two-sideband tuning solution is used when the 1875 and 2000 MHz-wide spectral windows are requested and the frequency coverage is greater than can be provided by a single-sideband solution. In this case, only the lower sidebands are used to arrange the different tunings i.e. the highest frequency in the lower sideband of tuning $n$ abuts against the lowest frequency in the lower sideband of tuning $n+1$. No overlap is assumed between these, but significant overlap will result elsewhere in the scan due to the presence of two sidebands. The effective bandwidth of 1875 MHz is assumed when arranging the 2000 MHz-wide TDM spectral windows.

The OT reports the frequencies of each spectral window and also displays them in the Visual Spectral Editor. Spectral windows corresponding to the same tuning are plotted with the same $y$-offset and displayed with the same colour, this being different for each tuning. Also shown is the requested frequency range for easy comparison with the actual frequency coverage.


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