Basic Information on selfcal

Task: selfcal
Purpose: Determine self-calibration of calibration gains.
Categories: calibration, map making

        SELFCAL is a MIRIAD task to perform self-calibration of
        visibility data.  Either phase only or amplitude and phase
        calibration can be performed.  The input to SELCAL are a
        visibility data file, and model images.  This program then
        calculates the visibilities corresponding to the model,
        accumulates the statistics needed to determine the antenna
        solutions, and then calculates the self-cal solutions.

Key: vis
        Name of input visibility data file. No default.

Key: select
        Standard uv data selection criteria.  See the help on select
        for more information.

Key: model
        Name of the input models.  Several models can be given, which
        can cover different channel ranges and different pointing and
        sources of the input visibility data.  Generally the model
        should be derived (by mapping and deconvolution) from the input
        visibility file, so that the channels in the model correspond to
        channels in the visibility file.  The units of the model MUST be
        Jy/pixel, rather than Jy/beam, and should be weighted by the
        primary beam.  The task DEMOS can be used to extract primary
        beam weighted models from a mosaiced image.  If no models are
        given, a point source model is assumed.
 
        Instead of a model image, you can also use a visibility model.
        Visibility models should have the same layout as the data and
        the same selections will be applied.
 
        NOTE: When you give SELFCAL a model, it will, by default, select
        the data associated with this model from the visibility data-
        set.  This includes selecting the appropriate range of channels
        and the appropriate pointing/source (if options=mosaic is used).
        If you use a point source model, if it YOUR responsibility to
        select the appropriate data.

Key: clip
        Clip level.  For models of intensity, any pixels below the clip
        level are set to zero.  For models of Stokes Q,U,V, or MFS
        I*alpha models, any pixels whose absolute value is below the
        clip level are set to zero.  Default is 0.

Key: interval
        The length of time, in minutes, of a gain solution.  Default is
        5, but use a larger value in cases of poor signal-to-noise, or
        if the atmosphere and instrument is fairly stable.

Key: nfbin
        The number of frequency bins. The default is 1. Use nfbin>1 to
        solve for variation across the band in the gains.
        Works best for uv files with a single spectral window, i.e.,
        after uvsplit.

Key: options
        This gives several processing options. Possible values are:
          amplitude  Perform amplitude and phase self-cal.
          phase      Perform phase only self-cal.
          mfs        This is used if there is a single plane in the
                     input model, which is assumed to represent the
                     image at all frequencies.  This should also be
                     used if the model has been derived from MFCLEAN.
          mmfs       Same as mfs, but with multiple input models,
                     each applying to a subset of the bandwidth.
                     For use with nfbin>1: models will be allocated to
                     bins based on their centre frequency.
          relax      Relax the convergence criteria. This is useful when
                     selfcal'ing with a very poor model.
          noscale    Do not scale the gains.  By default the gains are
                     scaled so that the rms gain amplitude is 1.  In
                     this way, the total flux is not constrained to
                     agree with the model. If you are using nfbin>1
                     you may need this to make the data follow the
                     spectral shape of the model.
          mosaic     This causes SELFCAL 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.
        Note that "amplitude" and "phase" are mutually exclusive.
        The default is options=phase.

Key: minants
        Data at a given solution interval is deleted if there are fewer
        than MinAnts antennae operative during the solution interval.
        The default is 3 for options=phase and 4 for options=amplitude.

Key: refant
        This sets the reference antenna, which is given a phase angle of
        zero.  The default, for a given solution interval, is the
        antenna with the greatest weight.

Key: flux
        If MODEL is blank, then the flux density (Jy) of a point source
        model can be specified here.  The default is 1.
        Additional spectral parameters can be specified as follows:
        Reference frequency in GHz, spectral index, up to two higher
        order alpha terms (as produced by uvsfit or uvfmeas with
        options=mfflux)

Key: offset
        This gives the offset in arcseconds of a point source model (the
        offset is positive to the north and to the east).  This
        parameter is used if the MODEL parameter is blank.  The default
        is 0,0.  The amplitude of the point source is chosen so that
        flux in the model is the same as the visibility flux.

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 parameter defaults to
        the linetype parameters used to construct the map.  If you wish
        to override this, or if the info is not in the header, or if you
        are using a point source model, this parameter can be useful.
 
Revision: 1.23, 2021/07/02 03:23:16 UTC