Miriad was originally designed for a telescope which simultaneously measured both spectral and continuum data using separate correlators. For SMA use, there is no real distinction between wideband (continuum) and spectral data. The Miriad distinction is not relevant. All the SMA data is treated initially as spectral data. Users can produce a ``continuum'' data by averaging the spectral data after certain corrections and calibrations. In addition, as has been mentioned in previous section, smalod supports a feature to resample the spectral data from high spectral resolution to low spectral resolution and from hybrid spectral resolution to uniform one. This is a good feature for calibration of antenna gains.
The SMA data in Miriad are formatted into a set of `spectral line windows' or `chunks' in the nomenclature of the SMA; each window consists of a number of channels separated by a fixed increment in sky frequencies. A task prthd prints a header of SMA data in Miriad format, which gives a glance of the SMA spectral data structure in Miriad.uvindex also gives the secptral configuration in addition to other information about the uv file.
It is quite common, when analyzing, plotting or mapping visibility data, that one will want to perform some averaging and selection of the desired channels, and one might wish to examine either spectral or the continuum data. The ability to select a range of spectral channels, to perform averaging, and to resample in velocity is provided by the line parameter - also called the linetype. If one's data contain multiple spectral windows, one should also refer to the next section SELECTION for more information on spectral window 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 is present. Since most SMA data is 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. This will be of no interest to SMA users since no wideband data are produced.
velocity - This gives spectral data, that have been resampled at equal increments in radio velocity (or equivalently frequencies). The resampling operation is a weighted average of spectral channels.
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 one gives 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. 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 center 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.
selects 10 output channels, being input spectral channels 1 to 10. Similarly
selects 10 output channels, starting at input spectral channel 2, 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
For SMA users, the following choice might be useful to form chunk averaged data for 24 spectral chunks and each chunks have 32 channels,
The above combination will produce chunk averaged (the central 26 channels) data for 24 spectral windows.
Finally a linetype of:
line=velocity,10,1.5,1.0,3.0would return 10 `velocity' channels with velocities centered 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).