MASK THRESHOLD
Keywords: imager

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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, mi-
        croK, or NATIVE.

        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 absolute 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
        default 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 ex-
        tends a little more than the inner quarter.  This is to avoid the  edges
        where sidelobes aliasing occurs.

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