Basic Information on uvmodel

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

        UVMODEL is a MIRIAD task that modifies a visibility dataset by
        combining or replacing it 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.
 
        As an example, 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 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.
          mosaic    Select only those visibilities whose observing
                    centre 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 centre selection
                    is performed.
          mfs       This is used if there is a single plane in the input
                    model, which is assumed to represent the data at all
                    frequencies.  This should also be used if the model
                    has been derived using MFCLEAN.
          zero      Use zero as the value of 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).  The flux can optionally be followed by i,q,u,v
        or the other polarisation mnemonics to indicate the polarisation
        type. The 3rd-6th parameter optionally specify the reference
        frequency in GHZ and the spectral index and higher order terms
        (as given by uvsfit or uvfmeas with options=mfflux).

Key: offset
        The RA and DEC offsets (arcsec) of the point source from the
        observing centre.  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.  This
        parameter may be used if you wish to override these defaults,
        or if the relevant information is not present in the header,

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.  The output file will contain
        only as many channels as there are planes in the model cube.
        The various uv variables that describe the windows are adjusted
        accordingly.  No default.
 
Revision: 1.12, 2021/06/03 07:09:31 UTC