Keywords: imager

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#### Implementation

In [rgb]1,0,0IMAGER, the variants of the `CLEAN` algorithms discussed above are coded as the following commands: magenta`HOGBOM`, magenta`CLARK`, magenta`MX`, magenta`SDI`, magenta`MULTI` and magenta`MRC`. All those commands work on two internal buffers containing the dirty beam and dirty image. Both buffers are created directly from uv table through the magenta`UV_MAP` command, or they can be loaded from files through the magenta`READ BEAM` and magenta`READ DIRTY` commands. The behavior of those commands is controlled through the following common red SIC variables:

Iterative search
  blue`CLEAN_POSITIVE`
    Number of positive clean components to be found before enabling the search for negative components. Default is 0.
  blue`CLEAN_GAIN`
    Loop gain. Default is 0.2, good compromise between stability and speed.

Stopping criteria
  blue`CLEAN_STOP`
    Compact way to specify the stopping criterium. Defaut is currently empty, but might be set to magenta`2 SIGMA` as a good general default.
  blue`CLEAN_NITER`
    Maximum number of clean components. Default is 0.
  blue`CLEAN_FRES`
    Maximum amplitude of the absolute value of the residual image. This maximum is expressed as a fraction of the peak intensity of the dirty image. Default value is 0.
  blue`CLEAN_ARES`
    Maximum amplitude of the absolute value of the residual image. This maximum is expressed in the image units (Jy/Beam). Default value is 0.
  blue`CLEAN_NKEEP`
    Minimum number of Clean components be- fore testing if Cleaning has converged. Default value is 70.

Support
  blue`BLC` and blue`TRC`
    Bottom Left Corner and Top Right Corner of a square support in pixel units. Default is 0, which means using only the inner quarter if no other support is defined.
  magenta`SUPPORT`
    A command that defines the support where to search for clean components. The support can be a Mask, or a Polygon. For a Polygon, the definition can be interactive, using the red GREG cursor. This definition can be stored in a file through the magenta`WRITE SUPPORT` command and read back in memory from the file with the magenta`SUPPORT` command. The polygon support definition is stored in the blue`SUPPORT%` structure. Command magenta`SUPPORT /MASK` instructs [rgb]1,0,0IMAGER to use the Mask instead of the polygon for the Clean support.
  magenta`MASK`
    Command magenta`MASK` is used to define a Mask-like support. This can be interactive, or automatic using a thresholding technique in command magenta`MASK THRESHOLD`. The computed Mask can be saved by command magenta`WRITE MASK`. The Mask can also be read by command magenta`READ MASK`. Command magenta`MASK USE` is equivalent to command magenta`SUPPORT /MASK`, and instructs [rgb]1,0,0IMAGER to use the Mask instead of the polygon for the Clean support.

Clean beam parameters
  blue`BEAM_SIZE`
    is a 3-element array that gives the FWHM size of the major and minor axes (in arcsec) and position angle (in degree) of the Gaussian used to restore the clean image from the clean component list. Default is all parameters at 0, meaning use the fit of the main lobe of the dirty image. Changing the default value of those parameters is dangerous. The beam size effectively used is available in blue`BEAM_FITTED`.

Other variables control specific aspects of a subclass of the `CLEAN` algorithm:

blue`CLEAN_NCYCLE`
  Maximum number of major cycles in all algorithms using this notion (magenta`CLARK`, magenta`MX`, magenta`SDI`). Default is 50.
blue`BEAM_PATCH`
  Size (in pixel units) of the dirty beam used to deconvolve the residual image in minor cycles. It is used in magenta`CLARK` and magenta`MRC` algorithms only. Default value is 0. This is for development only.
blue`CLEAN_SMOOTH`
  Smoothing factor between different scales in the MULTISCALE methods. Default value is sqrt(3).
blue`CLEAN_RATIO`
  Smoothing factor between different scales in the MRC method. Default value is 0, for which the code automatically derives the best power of 2 adequate for the current problem.

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