Content-type: text/html Manpage of imfit


Section: User Commands (1)
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imfit - Fit models to a given image dataset  




image analysis  


IMFIT is a Miriad task that fits model components to an image. If several image planes are given, each each is fitted separately. Optionally, the model or the residuals can be written out.

To get a good fit, it is important that you keep irrelevant pixels out of the fitting process, by using the region and/or the clip keywords. Also for multi-component fits, it is important to give reasonable estimates of the source parameters of the components (spar keyword).  


Name of the input image dataset. No default.
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.
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.
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
For example, to fit for two gaussians, use: `object=gaussian,gaussian'. There is no default.
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 initial estimates for each object a simply separated by a comma. 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.
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.

The optional output data-set. This is a miriad image. 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.
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 image.
           If an output is being created, the default is to make
           this the fitted model.

fit.for,v 1.7 2011/06/28 02:09:17 pteuben Exp $




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Time: 18:35:38 GMT, July 05, 2011