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Basic Information on mfclean
Task: mfclean
Purpose: Multi-frequency synthesis CLEAN.
Categories: deconvolution
MFCLEAN is a MIRIAD task to deconvolve a multi-frequency synthesis
image. It can perform either Clark or Hogbom iterations.
To achieve good results, the width of the beam should be at
least 3 times that of the region being cleaned. The
region being cleaned should be reasonably centred in the map, and
should have an appreciable guard band around it to the map edge (of
size comparable to the width of the region being cleaned).
To form a multi-frequency synthesis image and beam, use INVERTs
``mfs'' and ``sdb'' options. This will create
a map with one plane, and a beam with two planes (the normal dirty
beam, and the spectral dirty beam).
The result of MFCLEAN is a component image, consisting of two planes.
The first plane is the normal flux components. The second plane are
components of ``flux times spectral index'' (that is, I*alpha).
Use task MFSPIN to get a crude spectral index image from the output
of MFCLEAN.
The sign convention used for the spectral index, alpha, is that:
I(f) = I(f0) * (f/f0) ** alpha
MFCLEAN differs from CLEAN in a number of ways.
* Task CLEAN only requires that the beam is twice the size of the
region being cleaned whereas, for MFCLEAN, it is recommended that
the dirty beam be three times the size of the region being cleaned.
* MFCLEAN requires a guard band around the edge of the region being
cleaned.
* MFCLEAN does not have a Steer cleaning option, nor prussian hats.
Key: map
The input dirty map, which should have units of Jy/beam. No
default.
Key: beam
The input dirty beam. This should be formed using INVERT with
options=sdb. No default.
Key: model
An initial model of the deconvolved image. This could be the
output from a previous run of MFCLEAN. It must have flux units of
Jy/pixel. The default is no model (i.e. a zero map).
Key: out
The name of the output map. The units of the output will be
Jy/pixel. This file will contain the contribution of the input model.
It will consist of two planes, giving the flux density image and the
"flux times spectral index" image (also called the scaled flux
derivative image). No default.
Key: gain
The minor iteration loop gain. Two values can be given, the second
being the gain for the spectral components. If only one value is
given, the flux and spectral components use the same gain. The
default is 0.1.
Key: cutoff
MFCLEAN finishes when the absolute maximum residual falls below
CUTOFF. Default is 0.
Key: niters
The maximum number of minor iterations. MFCLEAN finishes when
abs(NITERS) minor iterations have been performed. Clean may finish
before this point, however, if NITERS is negative and the absolute
maximum residual becomes negative valued, or if the cutoff level
(as described above) is reached.
Key: region
This specifies the region to be Cleaned. See the Users Manual for
instructions on how to specify this. The default is generally
inadequate, and a smaller region should be explicitly specified.
Key: minpatch
The minimum patch size when performing minor iterations. Default
is 51, but make this larger if you are having problems with
corrugations. You can make it smaller when cleaning images which
consist of a pretty good dirty beam.
Key: speed
This is the same as the speed-up factor in the AIPS APCLN.
Negative values makes the rule used to end a major iteration more
conservative. This causes less components to be found during a
major iteration, and so should improve the quality of the Clean
algorithm. Usually this will not be needed unless you are having
problems with corrugations. A positive value can be useful when
cleaning simple point-like sources. Default is 0.
Key: mode
This can be either "hogbom", "clark" or "any", and
determines the Clean algorithm used. If the mode is "any", then
MFCLEAN determines which is the best algorithm to use. The default
is "any".
Key: log
Output log file containing a list of all the components. The log
file consists of 5 columns, being the iteration number, the x and
y pixel coordinate (in the output model; this goes from 1 to N),
the "I" component and the "I*alpha" component. The default is to not
create a log file.