Description
This tool function actually does gridding (and Fourier inversion if
needed) of visibility data to make an image. It allows calculation of
various types of image:
- observed
- Make the dirty image from the DATA column (default)
- model
- Make the dirty image from the MODEL_DATA column
- corrected
- Make the dirty image from the CORRECTED_DATA column
- residual
- Make the dirty image from the difference of the
CORRECTED_DATA and MODEL_DATA columns
- psf
- Make the point spread function
- singledish
- Make a single dish image
- coverage
- Make a single dish coverage image
- holography
- Make a complex holography image
- pb
- Make the primary beam as defined by setvp
Note the full imager equation is not used and so, for example, the
primary beam correction is not performed. Use
restore to get a residual image
using the full imager equation where primary beam correction is
performed.
A position shift can be applied when specifying the image parameters
with setimage. If a shift is specified then
the uvw coordinates are reprojected prior to gridding, and a phase
rotation is applied. If the image is a PSF then no phase shift is
applied but the uvw are recomputed. To see the effects of the uvw
reprojected, you can use the
plotuv function.
If desired, the full complex image (before conversion to stokes
I,Q,U,V) may be retained. Note that the image
tool cannot load a complex image directly. Instead, use the
imagecalc constructor
to take e.g. the real and imaginary parts of the image.
For making single dish and holography images, the data are convolved onto the
grid using a one of a number of options:
- gridfunction='SF'
- Circularly symmetric prolate spheroidal wavefunction.
This is always the same function in pixels. To get this to match to
the antenna primary beam, the optimum cellsize to use in constructing
the image is the antenna primary beam half-width-half-maximum times
1.20192.
- gridfunction='BOX'
- Nearest neighbor gridding.
- gridfunction='PB'
- The telescope primary beam is used as the
convolution function. This function is the same in arcseconds,
independent of the cellsize. This choice is optimum in the least
squares sense. To override the default choice of telescope primary beam
for a given telescope, use the function
setvp. Usually the default will be acceptable.
To make a reasonable approximation to the sky, one should divide
the type='singledish' image by the type='coverage' image, thresholding
at some level. For example:
include 'image.g';
imcov := image('scanweight'); imcov.view();
s:=0; imcov.statistics(s);
threshold := s.max / 10.0;
#
im:=imagecalc('sdimage',
pixels=spaste('scanimage[scanweight>', threshold,
']/scanweight[scanweight>', threshold, ']'))
im.view(raster=T, axislabels=T);
Arguments
| type |
in |
Type of output image |
|
|
Allowed: |
String |
|
|
Default: |
'observed' |
| image |
in |
Name of output image |
|
|
Allowed: |
String |
| compleximage |
in |
Name of output complex image |
|
|
Allowed: |
String |
| async |
in |
Run asynchronously in the background |
|
|
Allowed: |
Bool |
|
|
Default: |
!dowait |
Returns
Bool
Example
imgr.ft(model='3C273XC1.model', complist='3C273XC1.complist');
imgr.makeimage(type='residual', image='3C273XC1.residual')
imgr.makeimage(type='psf', image='3C273XC1.psf')
Fill in the MODEL_DATA column from Fourier transforming the model and
the componentlist. Make the residual image and write it to
3C273XC1.residual.
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