National Radio
Astronomy Observatory

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 i-th 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 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:

(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 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 W_k. The weight of the i-th 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∕σ²) per visibility (actually stored in the WEIGHT column).

Arguments

Example

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