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casa
$Rev:20696$
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casa
$Rev:20696$
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Functions | |
| def | widefield |
| def widefield.widefield | ( | vis = [''], |
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imagename = '', |
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outlierfile = '', |
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field = '', |
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spw = '', |
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selectdata = False, |
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timerange = '', |
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uvrange = '', |
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antenna = '', |
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scan = '', |
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mode = 'mfs', |
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niter = 500, |
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gain = 0.1, |
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threshold = '0.0Jy', |
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psfmode = 'clark', |
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ftmachine = '', |
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facets = 3, |
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wprojplanes = 64, |
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multiscale = [], |
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negcomponent = 0, |
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interactive = False, |
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mask = [], |
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nchan = 1, |
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start = 0, |
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width = 1, |
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imsize = [256, |
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cell = ['1.0arcsec', |
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| arcsec, | |||
phasecenter = '', |
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restfreq = '', |
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stokes = 'I', |
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weighting = 'natural', |
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robust = 0.0, |
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npixels = 0, |
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noise = '1.0Jy', |
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cyclefactor = 1.5, |
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cyclespeedup = -1, |
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npercycle = 100, |
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uvtaper = False, |
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outertaper = [''], |
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innertaper = ['1.0'], |
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restoringbeam = [''], |
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calready = False |
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| ) |
Wide-field imaging and deconvolution with selected algorithm
Wide-field imaging and deconvolution with selected algorithm:
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 on a size), see hints at the
end of the descriptions. For making images larger than about
5000x5000, the available memory must be larger than 2 Gbytes. For such
images therefore a computer with a 64-bit operating system may be
needed.
Keyword arguments:
vis -- Name of all input visibility files
default: none; example: vis='ngc5921.ms'
example: vis=['data01.ms', 'data02.ms']
imagename -- Pre-name of output images:
default: none; example: 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'; example: 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]; example: imsize=[500,500], or imsize=500
example for multiple fields: imsize=[(1000, 1000), (100, 100)]
cell -- Cell size (x,y)
default=['1arcsec,'1arcsec']
example: 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; example: 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'; example: stokes='IQUV';
Options: 'I','IV','IQU','IQUV'
niter -- Number iterations, set to zero for no CLEANing
default: 500; example: niter=500
gain -- Loop gain for CLEANing
default: 0.1; example: gain=0.1
threshold -- Flux level at which to stop CLEANing (units=mJy)
default: 0.0; example: threshold=0.0
mask -- Name(s) of mask image(s) used for CLEANing
default: '' example: 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: []; example: cleanbox=[110,110,150,145]
Note: 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; example: nchan=45
start -- Start channel, 0-relative
default=0; example: start=5
if mode='frequency' then a frequency value e.g start='1.4GHz'
width -- Channel width (value > 1 indicates channel averaging)
default=1; example: width=5
if mode='frequency' then a frequency value e.g width='10kHz'
step -- Step in channel number
default=1; example: 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
field='0~2'; field ids 0,1,2
field='0,4,5~7'; field ids 0,4,5,6,7
field='3C286,3C295'; field named 3C286 adn 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
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
example: 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'; example: weighting='uniform';
Options: 'natural','uniform','briggs','briggsabs','radial', 'superuniform'
robust -- 'briggs' and 'brigssabs' robustness parameter
default=0.0; example: 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
example: npixels=3
--- widefield controls
ftmachine -- Gridding method for the image;
ft (standard interferometric gridding).
wproject (wprojection algorithm for gridding)
default: wproject
wprojplanes -- Number w-projection planes to use for gridding
default: 256
example: 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
example: 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 (see below).
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; example: cyclefactor=4, but decreases speed considerably.
<cycle threshold = cyclefactor * max sidelobe * max residual>
cyclespeedup -- Cycle threshold doubles in this number of iterations
default: -1; example: cyclespeedup=500
--- MEM parameters (Experimental, not well-tested)
sigma -- Target image sigma
default: '0.001Jy'; example: sigma='0.1Jy'
targetflux -- Target flux for final image
default: '1.0Jy'; example: targetflux='200Jy'
constrainflux -- Constrain image to match target flux;
otherwise, targetflux is used to initialize model only.
default: False; example: constrainflux=True
prior -- Name of MEM prior images
default: ['']; example: 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; example: negcomponent=-1
scales -- Used for alg='multiscale'; set a number of scales or a vector
default: [0,3,10]; example: 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; example: 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
example: 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.
example: restoringbeam=['10arcsec'] circular Gaussian
FWHM 10" example:
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
======================================================================
HINTS ON RUNNING WIDEFIELD
1. Decide if the images will be specified directly in the
inputs or with an outlier file. For more than a few fields,
an outlier file more convenient.
Direct Method:
cell = ['1.0arcsec', '1.0arcsec']
imagename = ['M1_0','M1_1','M1_2]
imsize = [[1024,1024],[128,128],[128,128]]
phasecenter = ['J2000 13h27m20.98 43d26m28.0',
'J2000 13h30m52.159 43d23m08.02', 'J2000 13h24m08.16 43d09m48.0']
Text file method (in outlier.txt)
imagename = 'M1'
outlierfile = 'outlier.txt'
[phasecenter, imsize ignored]
Contents of outlier.txt
C 0 1024 1024 13 27 20.98 43 26 28.0
C 1 128 128 13 30 52.158 43 23 08.00
C 2 128 128 13 24 08.163 43 09 48.00
In both cases the following images will be made:
M1_0.image, M1_1.image, M1_2.image cleaned images
M1.0.model, M1_1.model, M1_2.model model images
M1.0.residual, M1_1.residual, M1_2.residual residual images
2. Wprojection: It is fastest to use wprojection without faceting.
ftmachine = 'wproject'
wprojplane = NN
The value of NN should be chosen as small as possible to reduce
execution time. The algorithm
NN = BMAX(klambda) * imagewidth (arcmin)^2 / 600, with a minimum
of 16, should be adequate.
3. Depending on the memory of the computer, a limit of about
5000x5000 may occur for example if a computer has 2Gbyte of
RAM. Also a 32-bit computer has a maximum limit of 2Gbyte
memory usable per process, irrespective of how much physical
RAM is present. Hence it is recommended to move to a 64-bit
computer with more than 2 GByte of RAM for >5000x5000 images
4. For data with extremely large 'w' values, i.e low frequency,
long baseline and very widefield image, the wprojection
convolution can be very large and either not fit in memory or
slow for processing. In these cases you should consider using
both ftmachine='wproject' and facets=xx where is 3.Definition at line 13 of file widefield.py.
References publish_summary.quantity, vla_uvfits_line_sf.verify, and task_widefield.widefield().
1.8.0