For ATCA use, there is no real distinction between wideband (continuum) and spectral data, and the Miriad distinction is not really relevant. All ATCA data (even channel-0 data) are treated as spectral data.
All the same, a Miriad visibility dataset can, in principle, contain multiple spectral and wideband correlators. The spectral data is described by a set of `spectral windows' (`IFs' or `IF channels' in AIPS terminology); each window consists of a number of channels separated by a fixed increment in sky frequency (though this increment can vary with time). Similarly there can be several measured wideband correlations, simply called wideband channels.
It is quite common, when analysing, plotting or mapping visibility data, that you will want to perform some averaging and selection of the desired channels, and you might wish to examine either the spectral or the wideband data. For the spectral data, if the channel number does not correspond reasonably directly with velocity (e.g. if Doppler tracking was not used), then it might be desirable to resample the spectral data at equal increments in velocity.
The ability to select a range of wideband or spectral channels, to perform averaging, and to resample in velocity is provided by the `line' parameter - also called the linetype. If your data contain multiple spectral windows, you should also refer to Section 5.6 for more information on spectral channel selection.
The linetype parameter consists of a string followed by up to four numbers. Defaults will be used for any trailing part of the linetype specification that is missing. The string can be one of:
velocity, but allows the velocity
parameters to be given using the optical definition. Note, however,
that the resampling operation is still in equal increments in frequency (or, equivalently, radio velocity). Because of the difference between
the radio and optical velocity definitions, equal increments in radio
velocity are not quite equal in optical velocity, and visa versa.
The velocity increment that you give is used as the optical
velocity increment of the first channel.
The accompanying four numbers are used to specify the range of input channels selected and averaged to produce the output channels. The four numbers are:
nchan, start, width, stepFor channel and wide linetypes, start, width and step are channel numbers (channels are numbered from 1 to N), whereas for velocity and felocity linetypes these values are in km/s (the velocity is relative to the rest frame - usually LSR). These values are
line=channel,10selects 10 output channels, being input spectral channels 1 to 10. Similarly
line=channel,10,8,1,2again selects 10 output channels, starting at input spectral channel 8, and skipping every second input channel. If you wanted to average together every pair of channels (rather than skipping it), you would use something like
line=channel,10,8,2,2Finally a linetype of:
line=velocity,10,1.5,1.0,3.0would return 10 `velocity' channels with velocities centred at 1.5, 4.5, 7.5, etc. km/s. Each channel would have a width of 1 km/s.
When using velocity, felocity or channel linetypes on datasets with multiple spectral windows, window selection, as described in the following sections, may be useful.
Some tasks require two linetypes, the first being the linetype of the data, and the second the linetype of a single reference channel (see e.g. invert). When specifying a reference linetype, you do not give the nchan (it is always 1) or step (it makes no sense for a single channel).