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).
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.
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,paHere "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.
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.
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 $