Basic Information on uvsfit

Task: uvsfit
Purpose: Fit point sources to a given vis file.
Categories: uv analysis

        UVSFIT is a Miriad task which fits model components to a visibility
        dataset. UVSFIT differs from UVFIT by fitting explicitly for the
        frequency dependence of source flux. Optionally the model or
        residual visibilities can be written out.
        UVSFIT can only handle a limited number of visibilities in fitting
        mode. You can reduce the data volume by averaging in frequency
        using the line parameter (apply bandpass first) or selecting a
        subset of the data. If you want to produce model or residual
        visibilities at full resolution for all the data, you can
        run UVSFIT again with all source parameters specified and fixed.
 

Key: vis
        Name of the input visibility file or files. No default.

Key: stokes
        Normal Stokes/polarisation parameter (e.g. i,q,u,v,ii etc).
        Only a single polarisation can be requested. The default is
        `ii' (i.e. Stokes-I for an unpolarised source).

Key: line
        Normal line-type processing with normal defaults.

Key: select
        Normal data selection. Default is all cross-correlation data.

Key: object
        This gives the object type that uvsfit fits for. Several objects
        can be given (the objects can be of the same type, or different),
        and minimum match is supported. Possible objects are
          point       A point source
          disk        An elliptical or circular disk.
          gaussian    An elliptical or circular gaussian.
          shell       The 2D projection of a thin, spherical shell.
          ring        A face-on, thin, elliptical or circular ring
        For example, to fit for a point source and gaussian, use:
        `object=point,gaussian'.

Key: spar
         This gives initial estimates of source parameters.  For
         each object given by the `object' keyword, either 3 (for
         point sources) or 6 (for disks and gaussians) values should be
         given. The values are as follows:
           Object Type             SPAR values
           -----------             -----------
            point                   flux,x,y
            gaussian                flux,x,y,bmaj,bmin,pa
            disk                    flux,x,y,bmaj,bmin,pa
            shell                   flux,x,y,bmaj
            ring                    flux,x,y,bmaj,bmin,pa
 
         Here "flux" is the total flux density of the component,
         "x" and "y" are the offset positions (in arcsec) of the object
         relative to the observing center, "bmaj" and "bmin" are the major
         and minor axes FWHM (in arcsec), and "pa" is the position angle
         of an elliptical component (in degrees). The position angle is
         measured from north through east.
         You must give initial estimates for all parameters for each object
         (this includes parameters that are redundant or meaningless,
         such as "bmin" and "pa" for components that are constrained to be
         circular).
 
         The more complex the set of objects being fitted for, the more
         important it is to give a good estimate of the source parameters.
         Generally the estimates of the source position should be accurate
         to the fundamental resolution (for point sources) or the size of
         the component (for extended sources).

Key: fix
         This gives a set a flag parameters, one parameter per source.
         Each parameter consists of a set of letters, which indicate
         which source parameters of a component are to be held fixed.
         These source parameters are fixed by the initial estimates
         given by the `spar' parameter.
         The letters corresponding to each source parameter are:
           f   The flux is fixed.
           x   The offset in RA is fixed.
           y   The offset in DEC is fixed.
           a   The major axis parameter is fixed.
           b   The minor axis parameter is fixed.
           p   The position angle parameter is fixed.
           c   The gaussian, disk or ring is circular (not elliptical).
         For a source where all source parameters vary, a dash (-)
         can be used for this parameter.
 
         For example "fix=fx,fc" indicates that the flux and RA offset
         is to be fixed for the first source, whereas the second source,
         (which is presumably a gaussian, disk or ring) has a fixed flux, and
         is circular.
 

Key: sspar
         This gives initial estimates of source spectral parameters.  For
         each object given by the `object' keyword, 3  values should be
         given. The values, for all object types, are as follows:
                      SSPAR values
                      -----------
                     alpha0,alpha1,alpha2
 
         Here "alpha0", "alpha1" and "alpha2" are defined in:
 
                   lfr = ln(frq/frq0)
                  alpha(frq) = alpha0 + lfr*(alpha1 + lfr*alpha2)
 
         giving the spectral index as a function of frequency so that
 
                   flux(frq) = flux*(frq/frq0)**alpha(frq)
 
         gives the component flux at frequency frq. Here frq0 is the reference
         frequency. If alpha0, alpha1, alpha2 are not given, values of zero
        are used.
        If any sspar values are given, all (three per source) must be given.

Key: sfix
         This gives a set a flag parameters for the spectral parameters,
         one parameter per source.  Each parameter consists of a set of
         digits, indicating which of the spectral terms are to be held fixed.
         These source parameters are fixed by the initial estimates
         given by the `sspar' parameter.
         The digits corresponding to each source parameter are:
           0   The spectral term alpha0 is fixed.
           1   The spectral term alpha1 is fixed.
           2   The spectral term alpha2 is fixed.
 
         For a source where all source parameters vary, a dash (-)
         can be used for this parameter.

Key: freqref
        A frequency in GHz used as the reference frequency for the flux
        and spectral index results.

Key: out
        Optionally, an output file will be written for each input file.
        If given, the number of output data-sets should equal the number
        of input visibility files, and either the model or residual
        visibilities can be saved.

Key: options
         Extra processing options. Several can be given, separated by commas.
         Minimum match is used. Possible values are:
           residual The residual visibilities are written to the output.
                    If an output is being created, the default is to make
                    this the fitted model.

Key: log
        Optionally, a log file will be written with the model parameters
        and their errors.  For each fitted source, four lines are written
        to the log: line 1 has source number, object type and three or six
        parameter values, depending on the object type; line 2 has the
        formal errors in each (0.0 if not a free parameter); line 3 has
        the values of all three spectral terms; line 4 has the formal
        errors in those terms (again 0.0 if not included in the fit).
 
Revision: 1.6, 2013/12/05 01:59:50 UTC