Documentation on uvlin

Task: 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 spectrum.
        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.

Key: vis
        The name of the input uv data set. No default.

Key: select
        The normal uv selection commands. The default is to select everything.

Key: line
        The normal uv linetype in the form:
          line,nchan,start,width,step
        The default is all channels. Note that if there are multiple
        spectral windows, the fitting process is performed on each window
        separately.

Key: chans
        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.

Key: out
        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.

Key: order
        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
        `relax' option.

Key: offset
        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 polynomial.
        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
        data-set.

Key: mode
        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
                         the default.
           '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
                         data-set.

Key: options
        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 interference.
           'twofit'      If either the OFFSET parameter is set, or the SUN
                         option invoked, this options instructs UVLIN to
                         perform a simultaneous polynomial fit for a emission
                         both at the phase centre and at the appropriate
                         offset.
           '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 corrections.
        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.