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Basic Information on 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
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
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
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
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.
Revision: 1.13, 2015/08/07 00:18:50 UTC
Generated by email@example.com on 21 Jun 2016