Implementation

In MAPPING, the variants of the CLEAN algorithms discussed above are coded as the following commands: HOGBOM, CLARK, MX, SDI, MULTI and 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 UV_MAP command, or they can be loaded from files through the READ BEAM and READ DIRTY commands. The behavior of those commands is controlled through the following common sic variables:

Iterative search

POSITIVE

Number of positive clean components to be found before enabling the search for negative components. Default is 0.

GAIN

Loop gain. Default is 0.2, good compromise between stability and speed.

Stopping criteria

NITER

Maximum number of clean components. Default is 500.

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 is 0.

ARES

Maximum amplitude of the absolute value of the residual image. This maximum is expressed in the image units (Jy/Beam). Default is 0.

Support

BLC and TRC

Bottom Left Corner and Top Right Corner of a square support in pixel units. Default is 0, meaning do not use such a square support.

SUPPORT

A command to interactively define the support where to search for clean components. This definition can be stored in a file through the WRITE SUPPORT command and read back in memory from the file with the SUPPORT command. The support definition is stored in the POLY sic structure. A side effect is that any definition of a polygon (e.g. through the GO FLUX command) will imply the definition of a support...

Clean beam parameters

MAJOR, MINOR and ANGLE

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 parameter is dangerous.

Other variables control specific aspects of a subclass of the CLEAN algorithms:

NMAJOR

Maximum number of major cycles in all algorithms using this notion (CLARK, MX, SDI). Default is 50.

BEAM_PATCH

Size (in pixel units) of the dirty beam used to deconvolve the residual image in minor cycles. It is used in all algorithms using the minor cycle notion. Default is 0, meaning *** ? JP ***

SMOOTH

*** ? JP ***