MASK THRESHOLD

        [ADVANCED\]MASK  THRESHOLD  Value  Unit  [GUARD  Guard]  [SMOOTH  Smooth
    Length]
      [REGIONS Nregion]

    Define  a Mask from thresholding the CLEAN image. If the CLEAN image is 3-D,
    the Mask will be 3-D.

    The method involves a first thresholding, optionally followed by a smoothing
    and a second thresholding to extend the support. The mask is the logical  OR
    of the Raw and Smooth masks.

    The  algorithm  to define the mask is controlled by 5 parameters, 4 of which
    are optional and specified as values after their corresponding keyword.  The
    arguments
        Value Unit
    indicate the threshold and its unit under which the mask is set to  0.   The
    unit can be %, SIGMA, NOISE, Jy, milliJy, microJy, K, milliK, microK, or NA-
    TIVE.

    Value  is not just a simple number: its interpretation depends on whether it
    has an explicit sigma (+ or -), in which case only values above of below are
    retained, of no sign at all, in which case comparisons are made on  the  ab-
    solute values.

    The  default  is  6  sigma. The noise is taken from the computed Clean noise
    (clean%gil%rms)  if  defined,  or  from  the  theoretical  noise  (clean  or
    dirty%gil%noise) if not.

        SMOOTH Smooth Length
            Smooth
    indicates  the  threshold  under  which  the  Smooth  Mask is set to 0 after
    smoothing.  There is no default. If keyword SMOOTH is not present, there  is
    no  smooth  mask.  This  smoothed threshold is in the same units as the main
    one.
            Length
    is the size of the smoothing gaussian to derive the  smooth  mask  from  the
    initial  mask  (in arcsec).  If set to *, or not specified, it takes the de-
    fault which is the Clean beam major axis.

        GUARD Guard
            Guard
    indicates the size of the guard band at edges where the mask is set to zero,
    in units of image size. The default is 0.18, i.e. the  mask  can  extends  a
    little  more than the inner quarter.  This is to avoid the edges where side-
    lobes aliasing occurs.

        REGIONS Regions
            Regions
    is the maximum number of separate regions retained in the mask.  The regions
    are ordered by decreasing size, and only the first Regions  ones  are  kept.
    Default is 0, which means keep all regions.