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Basic Information on uvlin
Purpose: Separate continuum and lines in a spectral data-set.
Categories: uv analysis
UVLIN separates line and continuum in a spectral visibility
data-set. It does this by fitting a low order polynomial to the
real and imaginary parts of the line free channels of each
This polynomial function is taken to represent the continuum, and
any deviation from this is taken to represent the line data.
Optionally a constraint can be added that the line shape is
constant, and its strength is proportional to the continuum
level. This is a good approximation in many recombination line
experiments. In this case, an iterative algorithm is used to
estimate the line shape and to optimally disentangle the line
and the continuum.
See the related tasks CONTSEN and CONTERR, which give estimates
of the noise amplification and the residual continuum.
The name of the input uv data set. No default.
The normal uv selection commands. The default is to select
The normal uv linetype in the form:
The default is all channels. Note that if there are multiple
spectral windows, the fitting process is performed on each window
This specifies the channel ranges that contain only continuum
(line free). It consists of a number of pairs, each pair giving
a start and end channel. These are relative to the channels
selected with the line parameter. The default is that all
channels are line-free, which is quite a good approximation if
the line is weak compared to the continuum.
The name of the output uv data-set. There is no default name.
The output can be either the line, continuum, or fitted line
shape, as specified by the options keyword. The default is to
write the line data.
Order of polynomial used to fit the continuum. The default is 1
(i.e. a linear fit). Possible values are 0 to 5. Also see the
An offset (in arcsec) that would shift the data to the center of
the dominant emission. Giving this shift allows UVLIN to reduce
the error involved in approximating the continuum as a
The shift information is only used in the fitting process -- the
output data is not shifted. The default is 0,0 (no shift).
The following options determine what is written to the output
This determines what is written out. The default is to write out
the line data. The following are possible values:
line The output data-set is the line data. This is
fit The output data-set is the fitted line shape data.
This is valid only when using the 'lpropc' option.
chan0 The output data-set is an average of all the
continuum data in a visibility record.
continuum The output data-set is the continuum data. This
has the same number of channels as the input
ratio The output data-set is the input data divided by
the fitted continuum data minus one.
This gives extra processing options. Several options can be given,
each separated by commas. They may be abbreivated to the minimum
needed to avoid ambiguity. Possible options are:
sun With this options, the OFFSET keyword is ignored,
and a shift appropriate for the Sun is determined
This can be useful to eliminate solar
twofit If either the OFFSET parameter is set, or the SUN
option invoked, this options instructs UVLIN to
perform a simultaneous polynomial fit for emission
both at the phase centre and at the appropriate
nowindow Normally uvlin processes the different windows
separately. This will usually give the best result
when the different windows are at quite different
frequencies. However when the windows are at
similar frequencies, or perhaps even abutting,
then it is better to perform a single fit over all
windows simultaneously. The "nowindow" option
causes uvlin to do a single fit over all windows.
lpropc The line shape is constant throughout the
data-set, and the intensity is proportional to the
continuum strength. This cannot be used together
with the 'twofit' option.
relax Normally UVLIN attempts to avoid overfitting the
data by reducing the order of the fit. It does
this if a significant number of channels are
flagged (more than 40% bad) or if it believes
there is inadequate data to perform the fit (less
than 5*(order+1) channels). You can overwrite this
conservatism, if you know what you are doing, with
the `relax' option.
The following options can be used to turn off calibration
The default is to apply any calibration present.
nocal Do not apply the gains table.
nopass Do not apply bandpass corrections. It is unwise
to turn off bandpass correction, as the continuum
estimation process will be confused by a bandpass
which is not flat.
nopol Do not apply polarization corrections.
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