image(levels, amplitudelevels, timescales, niter, gain, threshold, model, complist, image, residual, statsregion, statsout, algorithm, maskmodification)
Description
Makes a clean, self-calibrated image using the wide-field
algorithms.
For cleaning, the deconvolution is performed on the residual
image calculated from the visibility data currently selected. Hence
the first step performed in clean is to transform the current model or
models (optionally including a componentlist) to fill in the
MODEL_DATA column, and then inverse transform the residual
visibilities to get residual image for all facets. These residual
image are then cleaned using the corresponding point spread
function. This means that if the initial model is used as the starting
point for the deconvolution. Thus if you want to restart a clean,
simply set the model to the model that was previously produced by
clean.
The CLEAN deconvolution is joint in whatever Stokes parameters are
present. Thus it searchs for peaks in either I or I + | V| or
I + 
, the rationale for the latter two forms being
to be biased towards finding strongly polarized pixels first (these
forms are also the maximum eigenvalue of the coherency matrix). The
PSF is constrained to be the same in all polarizations (a feature of
this implementation, not of the Hamaker-Bregman-Sault formalism).
The clean algorithm is split into minor and major cycles. In the minor
cycles only the brightest points are cleaned, using a subset of the
point spread function. In the major cycle, the points thus found are
subtracted from the original visibilities. Note that aliasing can be
reduced by using the padding argument in
setoptions.
In the self-calibration phase, the data are self-calibrated to
reduce the rms difference between the current model and the observed
data. The antenna gain model may be either phase-only via the T-Jones or amplitude and phase via G-Jones (see
calibrater for more
details).
Self-calibration is invoked whenever the peak residual (in any field)
drops below the current level (as specified in the levels argument).
The best strategy is usually to set levels to initiate the
first self-calibration when close to but above the worst calibration
errors in the image, and thereafter down in steps of a factor of about
3. Usually at the high flux levels, only phase self-calibration is
required. One can switch to amplitude and phase self-calibration by
setting the argument amplitudelevel appropriately.
Note that the processing can be restarted from the existing images.
Note also that the number of facets may be changed on restart.
There are 2 options of masking, for defining regions in which to look
for CLEAN components, available. If the maskmodification
argument is set to 'auto', when the image is cleaned to the flux level of a
calibration stage, an automatic mask is made by thresholding the
restored image to the flux level reached. However if maskmodification is set to 'interactive' a viewer tool is started
prompting the user to draw the mask (See interactivemask).
Arguments
| levels |
|
Flux levels at which to self-calibrate e.g. 0.3Jy 0.1Jy 0.03Jy |
|
|
Allowed: |
Vector of strings |
|
|
Default: |
'0Jy' |
| amplitudelevels |
|
Flux level below which amplitude self-calibration is used |
|
|
Allowed: |
Quantity |
|
|
Default: |
'0Jy' |
| timescales |
|
Time scales for the self-calibrations e.g (60s 10s 10s) |
|
|
Allowed: |
Vector of strings |
|
|
Default: |
'10s' |
| niter |
|
Number of Iterations, set to zero for no CLEANing |
|
|
Allowed: |
Int |
|
|
Default: |
1000 |
| gain |
|
Loop Gain for CLEANing |
|
|
Allowed: |
Double |
|
|
Default: |
0.1 |
| threshold |
|
Flux level below which cleaning will stop |
|
|
Allowed: |
Quantity |
|
|
Default: |
'0Jy' |
| model |
|
Name of images |
|
|
Allowed: |
Vector of strings |
| complist |
|
Name of component list |
|
|
Allowed: |
String |
| image |
|
Names of restored images |
|
|
Allowed: |
Vector of strings |
| residual |
|
Names of restored images |
|
|
Allowed: |
Vector of strings |
| statsregion |
|
Region in which to calculate
statistics |
|
|
Allowed: |
Any valid region |
|
|
Default: |
unset |
| statsout |
|
Statistics record |
|
|
Allowed: |
Record |
|
|
Default: |
[=] |
| algorithm |
|
deconvolving algorithm |
|
|
Allowed: |
String |
|
|
Default: |
'wfclark' |
| maskmodification |
|
Masking update at every cal cycle from
'none', 'auto', 'interactive' |
|
|
Allowed: |
String |
|
|
Default: |
'none' |
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