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Single Continuum
This allows a maximum-bandwidth setup to be defined using only a single receiver band or frequency. Upon choosing a band, the OT will automatically set the spectral window frequencies to recommended defaults, such that the sensitivity is maximised. In Band 6, this involves avoiding the inner portions of the sidebands as these are somewhat (
20 per cent) noisier. See the Technical Handbook for more details. In receiver bands 3 to 8 two spws are placed in each sideband and the total bandwidth is 7.5 GHz. For Bands 9 and 10, 90-degree Walsh switching is enabled allowing four spws per sideband and a total bandwidth of 15 GHz. If this is selected, the image frequencies of each spw can be seen by clicking the ``Show image spectral windows'' button underneath each baseband.
It is possible to change the absolute positions of the spectral windows by changing a single parameter, the average frequency of the setup, but the relative separations remain the same. A low-resolution correlator mode (TDM) is chosen by default for each spectral window.
- Receiver Band: Selecting a band will set up default spectral window frequencies.
- Sky Frequency: This is the average frequency of the defined spectral windows and more-or-less corresponds to the value of the first local oscillator (LO1). There is a default for each band, but this can be changed here by entering a frequency in the observed (sky) frame. It is possible to reset to the band default frequencies using the button provided.
- Correlator Mode: Although the default is to use a low-spectral-resolution correlator mode (TDM), higher spectral resolution can also be selected. This will use FDM mode, still with a bandwidth per spw of 1875 MHz but with many more channels (4096 when using dual polarization). This can be useful for e.g. removing spectral lines during continuum data analysis but results in a much larger dataset and should only be used when really necessary.
- Rest Frequency: The rest frequency corresponding to the sky frequency is displayed for reference only. It corresponds to whichever source has been selected in the Targets table at the bottom of the Spectral Setup page.
The value of the Representative Frequency is displayed below the table of spectral windows. Its value will default to the centre of the chosen spectral window, but can be changed to another frequency within that spectral window if the transition of greatest interest is not at the centre.
Single Continuum is currently enforced for polarization mosaics and all solar observations. In these cases there is also no possibility to change the spw frequencies from their defaults.
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The ALMA OT Team, 2021 Feb 03