Parameter imagermode

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5.3.4 Parameter imagermode

This choose the mode of operation of clean, either as single-field deconvolution using image-plane major and minor cycles only (imagermode=’’), single-field deconvolution using Cotton-Schwab (CS) residual visibilities for major cycles (imagermode=’csclean’), or multi-field mosaics using CS major cycles (imagermode=’mosaic’).

In the default mode (imagermode=’’), the major and minor clean cycles work off of the gridded FFT dirty image, with residuals updated using the PSF calculation algorithm set by the psfmode parameter (§ 5.3.1). This method is not recommended for high dynamic range or high fidelity imaging applications, but can be significantly faster than CS clean. Note that for this option only, if mask=’’ (no mask or box set) then it will clean the inner quarter of the image by default.

The csclean choice specifies the Cotton-Schwab algorithm. This opens up the sub-parameters

magermode          =  ’csclean’        #  Options: ’csclean’ or ’mosaic’, ’’, uses psfmode
     cyclefactor    =        1.5        #  Controls how often major cycles are done. (e.g. 5 for
                                        #   frequently)
     cyclespeedup   =         -1        #  Cycle threshold doubles in this number of iterations

These options are explained below. In the CS mode, cleaning is split into minor and major cycles. For each field, a minor cycle is performed using the PSF algorithm specified in psfmode (§ 5.3.1). At major-cycle breakpoints, the points thus found are subtracted from the original visibilities. A fast variant does a convolution using a FFT. This will be faster for large numbers of visibilities. If you want to be extra careful, double the image size from that used for the Clark clean and set a mask to clean only the inner quarter or less (this is not done by default). This is probably the best choice for high-fidelity deconvolution of images without lots of large-scale structure.

Note that when using the Cotton-Schwab algorithm with a threshold (§ 5.3.12), there may be strange behavior when you hit the threshold with a major cycle. In particular, it may be above threshold again at the start of the next major cycle. This is particularly noticeable when cleaning a cube, where different channels will hit the threshold at different times.

ALERT: You will see a warning message in the logger, similar to this:

Zero Pixels selected with a Flux limit of 0.000551377 and a maximum Residual of 0.00751239

whenever it find 0 pixels above the threshold. This is normal, and not a problem, if you’ve specified a non-zero threshold. On the other hand, if you get this warning with the threshold set to the default of ’0Jy’, then you should look carefully at your inputs or your data, since this usually means that the masking is bad.

The option imagermode=’mosaic’ is for multi-field mosaics. This choice opens up the sub-parameters

imagermode          =   ’mosaic’   #  Use csclean or mosaic.  If ’’, use psfmode
     mosweight      =      False   #  Individually weight the fields of the mosaic
     ftmachine      =   ’mosaic’   #  Gridding method for the image
     scaletype      =    ’SAULT’   #  Controls scaling of pixels in the image plane.
     cyclefactor    =        1.5   #  change depth in between of  csclean cycle
     cyclespeedup   =         -1   #  Cycle threshold doubles in this number of iteration

These options are explained below.

   5.3.4.1 Sub-parameter cyclefactor
   5.3.4.2 Sub-parameter cyclespeedup
   5.3.4.3 Sub-parameter ftmachine
   5.3.4.4 Sub-parameter mosweight
   5.3.4.5 Sub-parameter scaletype
   5.3.4.6 The threshold revisited

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