Mode velocity

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5.2.5.4 Mode velocity

If mode=’velocity’ is chosen, then an output image cube with nchan channels will be created, with channels spaced evenly in velocity. Parameters are:

mode              = ’velocity’   #  Spectral image definition(mfs, channel, velocity,frequency)
   nchan          =         -1   #  Number of channels (planes) in output image
   start          =         ’’   #  Velocity of first image channel: e.g ’0.0km/s’(’’=default)
   width          =         ’’   #  Image channel velocity width: e.g ’-1.0km/s’(’’=default)
   interpolation  =  ’linear’   #  Spectral interpolation(nearest, linear, cubic)
   outframe       =         ’’   #  velocity reference frame of output image; ’’ =input
   veltype        =    ’radio’   #  velocity definition

Note that velocities are calculated with respect to the rest frequency in the MS or specified through the restfreq parameter (§ 5.2.7).

The velocity of the first output channel is given by start and spacing by width. Averaging is as in mode=’frequency’. The interpolation sub-parameter (§ 5.2.5.5) sets how channels are gridded into the image cube planes.

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 velocities of the channels in the MS. The defaults start=’’ and width=’’ will use the channel velocity 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.

Again, using the NGC5921 dataset as an example:

mode          =   ’velocity’
     nchan    =           21
     start    = ’1383.0km/s’
     width    =     ’10km/s’
     outframe =       ’LSRK’

Note that in this case the velocity axis runs forward, as opposed to the default channelization for ’channel’ or ’frequency’.

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