Documentation on imfit

Task: imfit
Purpose: Fit models to a given image dataset
Categories: image analysis

Key: in
        Name of the input image dataset. No default.

Key: region
        Normal region of interest. The fit is performed only in this
        region. This region should be modest in size. The default is
        the whole input image.

Key: clip
        Clip level. For input images of intensity, any pixels below the
        clip level are excluded from the fitting process. For other sorts
        of images (e.g. Stokes Q, U or V) pixels whose absolute values are
        below the clip level are excluded from the fit. The default is 0.

Key: object
        This gives the component types that IMFIT fits for. Several components
        can be given (the components can be of the same type, or different),
        and minimum match is supported. Possible objects are
          level       An offset (DC) level
          gaussian    An elliptical or circular gaussian.
          disk        An elliptical or circular disk.
          point       This is a short-hand for a gaussian with the
                      width of the point-spread-function.
          beam        This is a short-hand for a gaussian with a
                      peak value of 1 and located at the image centre.
                      This would typically be used when fitting a beam
                      pattern.
        For example, to fit for two gaussians, use:
        `object=gaussian,gaussian'. There is no default.

Key: spar
        This gives initial estimates of source parameters.  For
        each object given by the `object' keyword, either 1 (for
        the level) or 6 (for disks and gaussians) values should be
        given. The values are as follows:
          Object Type             SPAR values
          -----------             -----------
           level                   offset
           gaussian                amp,x,y,bmaj,bmin,pa
           disk                    amp,x,y,bmaj,bmin,pa
           point                   amp,x,y
           beam                    bmaj,bmin,pa
        Here offset is the offset level, "amp" is the peak value of
        the object, "x" and "y" are the offset positions (in arcsec) of
        the object relative to the reference pixel, "bmaj" and "bmin" are
        the major and minor axes FWHM (in arcsec), and "pa" is the position
        angle of an ellitpical component (in degrees). The position angle is
        measured from north through east.
        You should give initial estimates for all parameters for each object
        (this includes parameters that might seem redundant or meaningless,
        such as "bmin" and "pa" for components that are constrained to be
        circular). However if (and only if) you are fitting for a single
        object, IMFIT can derive an initial estimate for itself.

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 amplitude (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 or disk 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 amplitude and RA offset
        is to be fixed for the first source, whereas the second source,
        (which is presumably a gaussian or disk) has a fixed flux, and
        is circular.
        The default is to assume that everything can vary.

Key: out
        The optional output data-set. The default is not to create an
        output data-set. If an output dataset name is given, then
        either the model or residual image can be saved.

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