Parameter gridmode

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5.3.13 Parameter gridmode

The gridmode parameter is now provided to access more advanced deconvolution capabilities. The default gridmode=’’ is recommended for most cases.

The gridmode=’widefield’ option allows imaging in the wide-field regime where the W-term is not negligible. The CASA implementation allows both standard uv-plane faceting as well as the W-Projection algorithm ¹ or a combination of the two. Its sub-parameters are:

gridmode          = ’widefield’  #  Gridding kernel for FFT-based transforms, default=’’ None
     wprojplanes  =          1   #  Number of w-projection planes for convolution
     facets       =          1   #  Number of facets along each axis (main image only)

The wprojplanes parameter sets the number of pre-computed w-planes used for the W-Projection algorithm (wprojplanes=1 disables w-projection). The facets parameter sets the number of facets used. W-Projection, if used, is done for each facet. See § 5.3.18 below for more on wide-field imaging.

The gridmode=’aprojection’ option enables the gridding-based correction for RL beam-squint in off-axis systems (such as the VLA). The sub-parameters are:

gridmode    = ’aprojection’  #  Gridding kernel for FFT-based transforms, default=’’ None
   cfcache  = ’cfcache.dir’  #  Convolution function cache directory
   painc    =      360.0     #  Parallactic angle increment (degrees) for convolution function

Higher dynamic range imaging applications will require a-projection with correspondingly small (e.g. 10-degree) painc at the cost of speed. ALERT: This option is still under test and development, so be careful in its use.

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