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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 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 (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 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 extends
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 regions are ordered by decreasing size, and only the first Regions
ones are kept. Default is 0, which means keep all regions.