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Task: imfit Purpose: Fit models to a given image dataset Categories: 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). 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 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. 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. 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. 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 image. If an output is being created, the default is to make this the fitted model. Revision: 1.13, 2015/08/07 00:18:50 UTC