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Channel Selection, Averaging and Doppler Correction - UV Linetypes

Many Miriad tasks support on-the-fly selection and averaging of the channels to be processed. However, before launching into a description of this, we will review some Miriad history. Miriad was originally designed for a telescope which simultaneously measured both spectral and continuum data using separate correlators. Spectral data are generally narrowband, and the frequencies are defined to high precision. Doppler tracking is often employed. Continuum, or wideband, data have a much larger bandwidth, and frequency tolerances are not as great.`Channel-0' data (data formed by averaging all spectral channels together) are also treated as continuum data.

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:

channel

This gives raw or averaged spectral channels. This is generally the default if spectral data are present. As all ATCA data are treated as spectral data, this will be the most commonly used linetype.

wide

This gives raw or averaged wideband (continuum) data. This is the default if only wideband data are present. Probably this will be of no interest to ATCA users.

velocity

This gives spectral data, that have been resampled at equal increments in radio velocity (or equivalently frequency). The resampling operation is a weighted average of spectral channels. See Section 16.8 for more information.

felocity

This is like 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, step

For 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

nchan

The number of output channels produced. Generally it defaults to the maximum number of channels that can be produced from the input data. A value of zero can also be used to give you the default.

start

For channel and wide linetypes, the start value is the first input channel to be selected. For velocity and felocity linetypes, start is the centre velocity of the first output channel to be formed. The default value is 1 channel.

width

This value determines the width of the selected channels. For channel and wide linetypes, this gives the number of input channels to average together to produce a single output channel. For velocity and felocity linetypes, this gives the velocity width (in km/s) of the output channels. The default value is again 1 channel.

step

This parameter gives the increment between channels. For channel and wide linetypes, this gives the increment between selected input channels. For velocity and felocity linetype this gives the velocity increment between the output channels. This defaults to the same value as width.

For example

   line=channel,10

selects 10 output channels, being input spectral channels 1 to 10. Similarly

   line=channel,10,8,1,2

again 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,10,8,2,2

Finally a linetype of:

   line=velocity,10,1.5,1.0,3.0

would 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).