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imager.weight  Function
2.3.1 Apply additional weighting to the visibility weights
Description
Apply visibility weighting to correct for the local density of sampling in the uv
plane. The imaging weights are calculated on the fly when processing the data
and can be viewed by plotweights.
To correct for visibility sampling effects, natural, uniform, radial, and Briggs weighting are supported. These work as follows. Then:
 natural
 : minimizes the noise in the dirty image. The weight of the ith sample is
set to the inverse variance:
(2.4) where σ_{i} is the noise of the i’th sample.
 radial
 : approximately minimizes rms sidelobes for an eastwest synthesis array. The
weight of the ith sample is multiplied by the radial distance from the center of
the u,v plane:
(2.5)  uniform
 : For Briggs and uniform weighting, we first grid the inverse variance w_i
for all selected data onto a grid of size given by the argument npixels (default
to nx) and u,v cellsize given by 2∕fieldofview where fieldofview is
the specified field of view (defaults to the image field of view). This
forms the gridded weights W_k. The weight of the ith sample is then
changed:
(2.6) where W_k is the gridded weight of the relevant cell. It may be shown that this minimizes rms sidelobes over the field of view. By changing the field of view, one may suppress the sidelobes over a region different (usually smaller) than the image size.
 briggs: rmode=’norm’
 : The weights are changed:
(2.7) where:
(2.8) and R is the robust parameter. The scaling of R is such that R = 0 gives a good tradeoff between resolution and sensitivity. R takes value between 2.0 (close to uniform weighting) to 2.0 (close to natural).
 briggs: rmode=’abs’
 : The weights are changed:
(2.9) where R is the robust parameter and σ_{R} is the noise parameter.
For more details about Briggs (aka robust) weighting, see the Briggs thesis.
Note that this weighting is not cumulative since the imaging weights are calculated from the specified weight (function of noise; usually 1∕σ^{2}) per visibility (actually stored in the WEIGHT column).
Arguments
Inputs 
 
type 
 Type of weighting
 
 allowed:  string 

 Default:  natural  
rmode 
 Mode of briggs weighting
 
 allowed:  string 

 Default:  norm abs none 

noise 
 Noise used in absolute briggs weighting
 
 allowed:  any 

 Default:  variant 0.0Jy 

robust 
 Parameter in briggs weighting
 
 allowed:  double 

 Default:  0.0 

fieldofview 
 Field of view for uniform weighting
 
 allowed:  any 

 Default:  variant 0.0arcsec 

npixels 
 Number of pixels in the u and v directions
 
 allowed:  int 

 Default:  0 

mosaic 
 Individually weight the fields of a mosaic
 
 allowed:  bool 

 Default:  false 

async 
 Run asynchronously in the background
 
 allowed:  bool 

 Default:  false 

bool
Example
im.weight(type=’briggs’, rmode=’norm’, robust=0.5)
Applies Briggs (robust) weighting.
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