[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.
Gildas manager
2023-06-01