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Spectral Scan

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 (bandwidth $\ge$ 500 MHz). Using these parameters, the OT will then attempt to cover the requested frequency range using a maximum of five tunings. The OT will display a warning message if the requested frequency range can't be achieved because more than five tunings are required. It is generally not possible to cover the exact requested frequency range and the OT will usually provide more coverage than requested. Spectral averaging is available for the high-resolution modes, but will be the same for each spectral window of each tuning.

The OT tries to select the most efficient way of covering the requested range and will, generally speaking, use a single sideband for a relatively narrow range and two sidebands otherwise (this will activate 90-degree Walsh switching at Band 9 or 10). A single sideband is always used with the 500 or 1000-MHz spws. Tunings and spws are all slightly overlapped so as to avoid problems with edge channels. With two sidebands there is always a frequency range that is covered by more than one tuning and this unavoidably leads to non-uniform sensitivity across the scan.

The best way of seeing how the spectral windows are placed is by using the Spectral Visual Editor. Spectral windows corresponding to the same tuning are plotted with the same $y$-offset and displayed with the same colour. Also shown is the requested frequency range for easy comparison with the actual frequency coverage.

Spectral scans are currently not allowed for use with the Total Power Array.


next up previous contents
Next: Multiple velocities and tuning Up: Spectral Setup Previous: Single Continuum   Contents
The ALMA OT Team, 2021 Feb 03