Mode channel

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5.2.5.2 Mode channel

ALERT: Note that mode=’channel’ is intended as a shortcut to produce a cube based on the input MS channelization. It will be in the frame of the input MS. We recommend that users instead use the ’velocity’ and ’frequency’ modes which will produce cubes in other frames with more control of the cube spacing. These modes have defaults that will work from the MS spacing, reproducing the action of mode=’channel’.

If mode=’channel’ is chosen, then an image cube will be created. This is an expandable parameter, with dependent parameters:

mode              =  ’channel’   #  Spectral image definition(mfs, channel, velocity,frequency)
   nchan          =         -1   #  Number of channels (planes) in output image
   start          =          0   #  first input channel to use
   width          =          1   #  Number of input channels to average
   interpolation  =   ’nearest’   #  Spectral interpolation(nearest, linear, cubic)

The default nchan=-1 will automatically produce a cube with the number of channels needed to span the (regridded) spectral windows of the MS. If multiple MSs are used, the spectral frames of these need to be identical, e.g. LSRK. ALERT: This often results in extra blank channels at the beginning and end of the image cube, so it is usually more precise to specify nchan and start to get what you want. For best results, we also recommend ’nearest’ interpolation for the mode=channel.

The channelization of the resulting image is determined by the channelization in the MS of vis of the first spw specified (the “reference spw”). The actual channels to be gridded and used in the clean are selected via the spw parameter as usual. The resulting image cube will have nchan channels spaced evenly in frequency. The first output channel will be located at the frequency of channel start in the (first) reference spw (independent of what channels are selected using spw). If width> 1, then input MS channels with centers within a frequency range given by (width + 1)∕2 times the reference spw spacing will be gridded together (as in mode = ’mfs’ above) into the channels of the output image cube. The output channel spacing is thus given by width channels in the reference spw of the MS.

The interpolation sub-parameter (§ 5.2.5.5) sets how channels are gridded into the image cube planes. For ’nearest’, the channels in spw beyond the first are mapped into the nearest output image channel within half a channel (if any). Otherwise, the chosen interpolation scheme will be used. Image channels that lie outside the MS frequency range or have no data mapped to them will be blank in the output image, but will be in the cube.

See the example in § F.1 for using the ’channel’ mode to image a spectral-line cube. In this case, we use:

mode         = ’channel’
     nchan   =         46
     start   =          5
     width   =          1

which will produce a 46-channel cube starting with channel 5 of the MS with the same channel width as the MS. Note: the start channel is in reference to the channels in the MS, not the subset selected by spw.

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