Description

This is the main wide-field imaging/deconvolution task. It uses the wprojection method for a large field of view, can make many facets, and can include outlier fields. Several deconvolution algorithms are supported. Interactive cleaning is also supported.

For making large images (>2000 pixels on a size), see hints at the end of the descriptions. For making images larger than about 5000x5000 pixels, the available memory must be larger than 2 Gigabytes. For such images, a computer with a 64-bit operating system may be needed.

WARNING: The task widefield will be deprecated in CASA 5.4. 

 

Parameter descriptions

vis

Name of all input visibility files. Default: none. Examples: vis='ngc5921.ms'; vis=['data01.ms', 'data02.ms']

imagename

Pre-name of output images. Default: none. Examples: imagename='n5921'. If outlier fields are included, then imagename=['n5921', 'outlier1', outlier2'] and the first imagename is the wide-field image output images names are: n5921.clean, n5921.residual, n5921.model, n5921.interactive.mask

mode

Type of selection. Default: 'mfs'. Examples: mode='channel'; options: 'mfs', channel, velocity, frequency'

alg

Algorithm to use. Default: 'clark'; options: 'clark', 'hogbom','multiscale','entropy' Strongly advise 'clark'. 'multiscale' and 'entropy' well-tested.

imsize

Image pixel size (x,y). Default = [256,256]; Examples: imsize=[500,500], or imsize=500. Example for multiple fields: imsize=[(1000, 1000), (100, 100)]

cell

Cell size (x,y). Default=['1arcsec,'1arcsec']. Examples: cell=['0.5arcsec,'0.5arcsec'] or cell='0.5arcsec'

phasecenter

Direction position or the field for the image center. A list of the above is needed for multiple-fields. Default: '' -->field='0' as center. Examples: phasecenter='6'; phasecenter='J2000 19h30m00 -40d00m00'; phasecenter=['J2000 19h30m00 -40d00m00', 'J2000 19h57m00 40d00m00'] for wide-field, plus one outlier field.

stokes

Stokes parameters to image. Default='I'. Examples: stokes='IQUV'; options: 'I','IV','IQU','IQUV'

niter

Number iterations, set to zero for no CLEANing. Default: 500. Examples: niter=500

gain

Loop gain for CLEANing. Default: 0.1. Examples: gain=0.1

threshold

Flux level at which to stop CLEANing (units=mJy). Default: 0.0. Examples: threshold=0.0

mask

Name(s) of mask image(s) used for CLEANing. Default: ''. Examples: mask='orion.mask' (Number of mask fields must equal number of imaged fields)

cleanbox

List of [blc-x,blc-y,trc-x,trc-y] values. Default: [ ]. Examples: cleanbox=[110,110,150,145]; This can also be a filename with clean values: fieldindex blc-x blc-y trc-x trc-y; cleanbox = 'interactive' is very useful.

 

Data selection

nchan

Number of channels to select. Default: 1. Examples: nchan=45

start

Start channel, 0-indexed. Default=0. Examples: start=5; if mode='frequency' then a frequency value e.g., start='1.4GHz'

width

Channel width (value > 1 indicates channel averaging). Default=1. Examples: width=5; if mode='frequency' then a frequency value e.g., width='10kHz'

step

Step in channel number. Default=1. Examples: step=2

field

Select field using field id(s) or field name(s). [run listobs to obtain the list id's or names] Default: ''=all fields. If field string is a non-negative integer, it is assumed a field index otherwise, it is assumed a field name. Examples: field='0~2',field='0~, field ids 0,1,2; field='0,4,5~7', field ids 0,4,5,6,7; field='3C286,3C295', field named 3C286 and 3C295; field = '3,4C*', field id 3, all names starting with 4C; example for multiple ms in vis parameter: field=['0~2', '1,2']

spw

Select spectral window/channels. Default: ''=all spectral windows and channels. Examples: spw='0~2,4', spectral windows 0,1,2,4 (all channels); spw='<2', spectral windows less than 2 (i.e. 0,1); spw='0:5~61', spw 0, channels 5 to 61; spw='0,10,3:3~45', spw 0,10 all channels, spw 3, channels 3 to 45; spw='0~2:2~6', spw 0,1,2 with channels 2 through 6 in each; spw='0:0~10;15~60', spectral window 0 with channels 0-10,15-60; spw='0:0~10,1:20~30,2:1;2;3', spw 0, channels 0-10, spw 1, channels 20-30, and spw 2, channels, 1,2 and 3; for multiple ms in vis parameter: spw=['0,10,3:3~45', '<2']

timerange

Select time range subset of data (not implemented yet). Default='' meaning no time selection. Examples: timerange='YYYY/MM/DD/HH:MM:SS.sss'; timerange='< YYYY/MM/DD/HH:MM:SS.sss'; timerange='> YYYY/MM/DD/HH:MM:SS.sss'; timerange='ddd/HH:MM:SS.sss'; timerange='< ddd/HH:MM:SS.sss'; timerange='> ddd/HH:MM:SS.sss'

restfreq

Specify rest frequency to use for image. Default='' (i.e., try to use the restfreq specified in the visibility data).

 

Weighting

weighting

Weighting to apply to visibilities. Default='natural'. Examples: weighting='uniform'; options: 'natural', 'uniform', 'briggs', 'briggsabs', 'radial', 'superuniform'

robust

'briggs' and 'briggsabs' robustness parameter. Default=0.0. Examples: robust=0.5; options: -2.0 to 2.0; -2 (uniform)/+2 (natural)

npixels

Number of pixels to determine uv-cell size for weight calculation. Used with 'superuniform' or 'briggs' weighting schemes. Examples: npixels=3

ftmachine

Gridding method for the image. Options: 'ft' (standard interferometric gridding) or 'wproject' (wprojection algorithm for gridding). Default: 'wproject'

wprojplanes

Number of w-projection planes to use for gridding. Default: 256. Examples: wprojplanes=64; Good value = BMAX(klambda) * Map width(arcmin)^2 / 600

facets

Number of facets along one axis on central image. Image is divided in facets x facets rectangles. Default: 1. Examples: facets=3 makes 3x3 images to cover the field; if ftmachine = 'ft', only faceting is used; if ftmachine = 'wproject', both wplanes and faceting can be used.

cyclefactor

Change the threshold at which the deconvolution cycle will stop and degrid and subtract from the visibilities. For bad PSFs, reconcile often (cyclefactor=4 or 5); For good PSFs, use cyclefactor 1.5 to 2.0. Default: 2.5.  Examples: cyclefactor=4, but decreases speed considerably; 

cyclespeedup

Cycle threshold doubles in this number of iterations. Default: -1. Examples: cyclespeedup=500

 

MEM parameters (Experimental, not well-tested)

sigma

Target image sigma. Default: '0.001Jy'. Examples: sigma='0.1Jy'

targetflux

Target flux for final image. Default: '1.0Jy'. Examples: targetflux='200Jy'

constrainflux

Constrain image to match target flux, otherwise targetflux is used to initialize model only. Default: False. Examples: constrainflux=True

prior

Name of MEM prior images. Default: ['']. Examples: prior='source_mem.image'

 

Multi-scale parameters (Experimental, not well-tested)

negcomponent

Stop component search when the largest scale has found this number of negative components; -1 means continue component search even if the largest component is negative. Default: 2. Examples: negcomponent=-1

scales

Used for alg='multiscale'; set a number of scales or a vector. Default: [0,3,10]. Examples: scales=[0.0,3.0,10.0, 30]

 

Interactive masking

npercycle

When cleanbox is set to 'interactive', this is the number of iterations between each clean to update mask interactively. However, this number can be adjusted during execution.

 

uvtaper

Apply additional uv tapering of the visibilities. Default: uvtaper=False. Examples: uvtaper=True

uvtaper=True expandable parameters

outertaper

uv-taper on outer baselines in uv-plane [bmaj, bmin, bpa] taper Gaussian scale in uv or angular units. NOTE: uv taper in (klambda) is roughly on-sky FWHM(arcsec/200). Default: outertaper=[ ] (no outer taper applied). Examples: outertaper=['5klambda'] circular taper FWHM=5 kilo-lambda; outertaper=['5klambda','3klambda','45.0deg']; outertaper=['10arcsec'] on-sky FWHM 10"; outertaper=['300.0'] default units are meters in aperture plane

innertaper

uv-taper in center of uv-plane. NOT YET IMPLEMENTED

 

restoringbeam

Output Gaussian restoring beam for CLEAN image [bmaj, bmin, bpa] elliptical Gaussian restoring beam. Default units are in arc-seconds for bmaj,bmin, degrees for bpa.  Default: restoringbeam=[ ] use PSF calculated from dirty beam. Examples: restoringbeam=['10arcsec'] circular Gaussian FWHM 10"; restoringbeam=['10.0','5.0','45.0deg'] 10"x5" at 45 degrees

calready

If True will create scratch columns if they are not there. And after clean completes the predicted model visibility is from the clean components are written to the MS.

async

Run asynchronously. Default = False. (do not run asychronously)