Documentation on uvmodel

Task: uvmodel
Purpose: Add, subtract, etc, a model from a uv data set.
Categories: uv analysis

        UVMODEL is a MIRIAD task which modifies a visibility dataset by a model.
        Allowed operations are adding, subtracting, multiplying, dividing and
        replacing. The model is specified in the image domain, so that its
        Fourier transform is first computed before application to the
        visibilities. The model may be either an image (e.g., a CLEAN
        component image) or a point source.

        An example is as follows. UVMODEL could be used to remove CLEAN
        components from a visibility data file.  The residual data base could
        then be examined for anomalous points, which could in turn be clipped.
        UVMODEL could then be reapplied to add the CLEAN components back into
        the visibility data base for re-imaging. 

Key: vis
        Input visibility data file. No default

Key: model
        Input model cube. The default is a point source model.
        This will generally be a deconvolved map, formed from the visibility
        data being modified. It should be made with ``channel'' linetype.
        The model should have units of JY/PIXEL and be weighted by the primary
        beam. The task DEMOS can be used to extract primary beam weighted
        models from a mosaiced image.

Key: select
        The standard uv selection subcommands. The default is all data.

Key: options
        This gives extra processing options. Several values can be given
        (though many values are mutually exclusive), separated by commas.
        Option values can be abbreviated to uniqueness.
        Possible options are (no default):
          add       Form: out = vis + model
          subtract  Form: out = vis - model
          multiply  Form: out = vis * model
          divide    Form: out = vis / model
          replace   Form: out = model
          flag      Form: out = vis, but flag data where the difference
                    between vis and model is greater than "sigma" sigmas.
          unflag    Unflag any flagged data in the output.
          autoscale Adjust the scale of the model to minimise the difference
                    between the model and the visibility.
          apriori   Use flux from flux table or data file planet info.
          imhead    Much ``header'' information in the uv file is ignored -- the
                    model information is used instead. In particular, the
                    observing center of the uv file is taken to be the reference
                    pixel of the model. By default, if the reference pixel
                    and the uv data observing center are different, a phase
                    shift is applied to the model visibilities to align them.
                    The ``imhead'' option prevents this shifting.
          selradec  This causes UVMODEL to select only those visibilities
                    whose observing center is within plus or minus three
                    pixels of the model reference pixel. This is needed
                    if there are multiple pointings or multiple sources in
                    the input uv file. By default no observing center
                    selection is performed.
          polarized The source is polarized. By default the source is
                    assumed to be unpolarized. For a polarized source,
                    UVMODEL cannot perform polarization conversion. That is,
                    if the model is of a particular polarization, then the
                    visibility file should contain that sort of polarization.
                    For example, if the model is Stokes-Q, then the visibility
                    file should contain Stokes-Q.
          mfs       This is used if there is a single plane in the input
                    model, which is assumed to represen t the data at all
                    frequencies. This should also be used if the model has
                    been derived using MFCLEAN.
         zero      Use the value zero for the model if it cannot be 
                    calculated. This can be used to avoid flagging the 
                    data in the outer parts of the u-v-plane when subtracting
                    a low resolution model.
        The operations add, subtract, multiply, divide, replace and flag are
        mutually exclusive. The operations flag and unflag are also mutually
        exclusive.

        The unflag option should be used with caution. Data in the output
        may still be flagged, if it was not possible to calculate the
        model.

Key: clip
        Clip level. Pixels in the model below this level are set to zero.
        The default is not to perform any clipping.

Key: flux
        If MODEL is blank, then the flux (Jy) of a point source model should
        be specified here. Also used as the default flux in the apriori
        option. The default is 1 (assuming the model parameter is not given).

Key: offset
        The RA and DEC offsets (arcseconds) of the point source from the
        observing center. A point source to the north and east has positive
        offsets. Defaults are zero.

Key: line
        The visibility linetype to use, in the standard form, viz:
          type,nchan,start,width,step
        Generally if there is an input model, this defaults to the linetype
        parameters used to construct the map. For a point source or planet
        model, the default is all channels. If you wish to override these
        defaults, or if the info is not present in the header, this parameter
        can be useful.

Key: sigma
        For options=flag, UVMODEL flags those points in the output that
        differ by more than "sigma" sigmas. The default is 100.

Key: out
        Output visibility data file name. No default. The output file will
        contain only as many channels as there are planes in the model
        cube. The various uv variables which describe the windows are
        adjusted accordingly.