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 (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.