Description
Apply visibility weighting to correct for the local density of
sampling in the uv plane. The imaging weights are written into a Table
column called IMAGING_WEIGHT, which may be plotted using
plotweights. In addition this
columns may be accessed directly using either the
table or
ms modules.
To correct for visibility sampling effects, natural, uniform (the
default), radial, and Briggs weighting are supported. These work as
follows. Then:
- natural
- : minimizes the noise in the dirty image. The weight of
the i-th sample is set to the inverse variance:
wi =
|
(1.25) |
where
is the noise of the i'th sample.
- radial
- : approximately minimizes rms sidelobes for an east-west synthesis
array. The weight of the i-th sample is multiplied
by the radial distance from the center of the u, v plane:
wi = wi
|
(1.26) |
- uniform
- : For Briggs and uniform weighting, we first grid the inverse
variance wi for all selected data onto a grid of size given by the
argument npixels (default to nx) and u,v cell-size given by
2/fieldofview where fieldofview is the specified field of view
(defaults to the image field of view). This forms the gridded weights
Wk. The weight of the i-th sample is then changed:
wi =
|
(1.27) |
where Wk 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:
wi =
|
(1.28) |
where:
f2 =
|
(1.29) |
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:
wi =
|
(1.30) |
where R is the robust parameter and
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 sigma (expected noise) per visibility
(actually stored in the SIGMA column).
Arguments
type |
|
Type of weighting |
|
|
Allowed: |
String: 'uniform'|'natural'|'briggs'|'radial' |
|
|
Default: |
uniform |
rmode |
|
Mode of briggs weighting |
|
|
Allowed: |
String: 'norm'|'abs'|'none' |
|
|
Default: |
none |
noise |
|
Noise used in absolute briggs weighting |
|
|
Allowed: |
Quantity |
|
|
Default: |
'0.0Jy' |
robust |
|
Parameter in briggs weighting |
|
|
Allowed: |
Double: range
-2.0 to 2.0 |
|
|
Default: |
0.0 |
fieldofview |
|
Field of view for uniform weighting |
|
|
Allowed: |
Quantity |
|
|
Default: |
'0arcsec' |
npixels |
|
Number of pixels in the u and v directions |
|
|
Allowed: |
Int |
|
|
Default: |
0 |
async |
|
Run asynchronously in the background |
|
|
Allowed: |
Bool |
|
|
Default: |
!dowait |