Basic Information 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.
           ratio       The output data-set is the input data divided by
                       the fitted continuum data minus one.

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 emission
                       both at the phase centre and at the appropriate
                       offset.
           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
        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.