Outlier fields

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5.3.18.1 Outlier fields

When using wide-field imaging, the position and image size of any independent images must be specified. Those positions will be used to add additional cleaning components to strong sources that may reside in that area and influence the central image.

There are a two options to specify the outlier fields:

Direct listing of fields The outlier field directions are provided via their centers (phasecenter parameter), and their sizes as a second entry in the imsize parameter, e.g. 128 pixels in the example below. clean will derive two additional images and their names are to be provided in the imagename field that will then be a list of the main field name plus the outlier field names:

vis           = ’wfield.ms’                #  name of input visibility file
imagename=[’n5921’,’outlier1’,’outlier2’]  #  Pre-name of output images
outlierfile   = ’’                         #  Text file with image names, sizes, centers
mask          = [[’image_setup.rgn’],[’’],[’’]]
imsize        = [[2048,2048],[128,128],[128,128]]   #  Image size in pixels (nx,ny)
cell          = ’1.0arcsec’                #  The image cell size in arcseconds [x,y].
phasecenter   = [’’,’J2000 13h27m20.98 43d26m28.0’, ’J2000 13h30m52.158 43d23m08.00’]

Outlier file For many outlier fields, it may be easier to setup the main interface to clean for the main field only and list outlier fields in an additional outlierfile:

imagename=’n5921’
outlierfile = ’outliers.txt’
imsize=[1024,1024]
phasecenter = ’’

outliers.txt provides all outlier fields with a syntax that is similar to the direct input, but separated by field. Below is an example for an outlierfile:

#content of outliers.txt
#
#outlier field1
imagename=’outlier1’
imsize=[512,512]
phasecenter = ’J2000 13h30m52.15 43d23m08.0’
mask=’box[[245pix,245pix],[265pix,265pix]]’
#
#outlier field2
imagenname=’outlier2’
imsize=[512,512]
phasecenter = ’J2000 13h24m08.16 43d09m48.0’

The syntax rules for the outlier files are:

each field must begin with imagename followed by
imsize and phasecenter must be given
optionally a mask can be provided. The mask parameter follows the CASA region file convention (Chapter D) or can be a mask file or LEL string.

The older AIPS-style convention (and box definition) that was used in CASA 3.2 and earlier is still supported in CASA 3.3 but will be deprecated for CASA 3.4 and higher.

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