Mode frequency

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5.2.5.3 Mode frequency

For mode=’frequency’, an output image cube is created with nchan channels spaced evenly in frequency.

mode              = ’frequency’   #  Spectral image definition(mfs, channel, velocity,frequency)
   nchan          =          -1   #  Number of channels (planes) in output image
   start          =          ’’   #  Frequency of first image channel: e.q. ’1.4GHz’(’’=default)
   width          =          ’’   #  Image channel frequency width: e.g ’1.0kHz’(’’=default)
   interpolation  =    ’linear’   #  Spectral interpolation(nearest, linear, cubic)
   outframe       =          ’’   #  velocity frame of output image

The frequency of the first output channel is given by start and spacing by width. Output channels have width also given by width. The sign of width determines whether the output channels ascend or descend in frequency. Data from the input MS with centers that lie within one-half an input channel overlap of the frequency range of ±width∕2 centered on the output channels are gridded together.

The defaults are designed to safely choose output cube channels to span the input MS(s). The default nchan=-1 will choose the number of channels needed to span the frequencies of the channels in the MS. The defaults start=’’ and width=’’ will use the channel frequency and width of the first channel of the first specified spectral window selected in spw. ALERT: As in “channel” mode, this is currently the first channel of the first spw, not the first channel selected from that spw.

The interpolation sub-parameter (§ 5.2.5.5) sets how channels are gridded into the image cube planes.

Using the NGC5921 dataset as an example:

mode          =   ’frequency’
     nchan    =            21
     start    = ’1412.830MHz’
     width    =       ’50kHz’
     outframe =        ’LSRK’

would produce a 21-channel output cube with 50 kHz wide channels rather than the default channelization of the MS (24.4 kHz).

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