Documentation on clean

Task: clean
Purpose: Apply Hogbom, Clark or Steer CLEAN algorithm to a map
Categories: deconvolution

        CLEAN is a MIRIAD task, which performs a hybrid Hogbom/Clark/Steer Clean
        algorithm, which takes a dirty map and beam, and produces an output
        map which consists of the Clean components. This output can be
        input to SELFCAL to self-calibrate visibilities, or input to RESTOR
        to produce a "clean" image. Optionally CLEAN can take as one of
        its inputs a model of the deconvolved image. This model could be
        from a previous CLEAN run, or from other deconvolution tasks
       (e.g. MAXEN).

Key: map
        The input dirty map, which should have units of Jy/beam. No
        default. 

Key: beam
        The input dirty beam. No default

Key: model
        An initial model of the deconvolved image. This could be the
        output from a previous run of CLEAN, or the output of any of the
        deconvolution tasks (e.g. MAXEN). 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 also should have a different name to the input model (if any).
        It can be input to RESTOR, CLEAN (as a model, to do more cleaning),
        or SELFCAL (for self-calibrating visibility data).

Key: gain
        The minor iteration loop gain. Default is 0.1.

Key: options
        Extra processing options. Several can be given, separated
        by commas. Minimum match is used. Possible values are:
          negstop   Stop when the first negative component is encounters.
                    This does not apply when using Steer iterations.
          positive  Apply a positivity constraint. This constrains the
                    component image to be non-negative. A side-effect of this
                    is that CLEAN will stop iterating if it cannot continue
                    to ensure this. This does not apply when using Steer
                    iterations.
          asym      The beam is asymmetric. By default CLEAN assumes the
                    beam has a 180 degree rotation symmetry, which is the
                    norm for beams in radio-astronomy.
          pad       Double the beam size by padding it with zeros. This
                    will give you better stability with Clark and Steer
                    modes if you are daring enough to CLEAN more than the
                    inner quarter of the dirty image.

Key: cutoff
        CLEAN finishes either when the absolute maximum residual falls
        below CUTOFF, or when the criteria described below is
        satisfied. The default CUTOFF is 0. 

Key: niters
        The maximum number of minor iterations. The default is 250, which
        is too small for all but the simplest of images. CLEAN will stop
        when either the maximum number of iterations is performed, or the
        cutoff (see above) is reached, or if options=negstop was given and
        a negative component was found, or if options=positive was given,
        and no more positive components could be found.

Key: region
        This specifies the region to be Cleaned. See the Users Manual for
        instructions on how to specify this. The default is the inner
        quarter of all planes.

Key: phat
        Cornwells prussian hat parameter. When cleaning extended sources,
        CLEAN may produce a badly corrugated image. This can be suppressed
        to some extent by cleaning with a dirty beam which has had a spike
        added at its center (i.e. a beam that looks like a prussian hat).
        PHAT gives the value of this spike, with 0 to 0.5 being good
        values. Default is 0 (but use a non-zero value for extended
        sources). 

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", "steer" or "any", and
        determines the Clean algorithm used. If the mode is "any", then
        CLEAN determines which is the best algorithm to use. The default
        is "any". 

Key: clip
        This sets the relative clip level in Steer mode. Values are
        typically 0.75 to 0.9. The default is image dependent.