Description
dragon can perform wide-field imaging as needed to overcome the
non-coplanar baselines effect for the VLA and other non-coplanar
arrays. It is built upon the imager
and calibrater tools for
imaging and calibration. The extra capability of dragon is mainly in
providing a simple and effective interface for self-calibration.
In wide-field imaging (e.g. VLA observing at 327MHz), the geometry of
the array as seen from the source varies significantly over the field
of view. This means that the relationship between sky brightness and
visibility is no longer a simple 2D Fourier transform. However, the
true relationship can be approximated by a series of tangent plane
approximations in which a 2D Fourier transform is used over a small
fraction of the image (usually referred to as one facet). Cleaning
using a point spread function is limited to one facet at a time, and
the cross-subtraction of points from one facet to another is
accomplished by using a complete Fourier transform to the visibility
domain.
Compared to similar programs in other packages (i.e. SDE dragon
and the AIPS IMAGR), dragon has a number of important advantages:
- The conversion to a common tangent plane is performed in the
visibility plane, and so a single output image is produced with no
extra effort (this was first suggested by Sault, Staveley-Smith and
Brouw, 1994).
- The decomposition of the image plane into facets may be changed
during the processing, for example, starting with few facets and
then proceeding to more facets at deeper levels of cleaning.
- The current clean image is displayed after each clean cycle.
- Self-calibration is performed as part of the imaging loop, thus
saving time (this is also true of the SDE dragon but not of AIPS IMAGR).
- The image may be supplemented by
componentmodels, which can be
generated using the interactive imagerfitter.
These componentmodels follow a separate path through the processing,
at high precision.
- dragon can be customized by attaching
commands using the AIPS++ plug-in system. See the file
code/trial/apps/imager/dragon_standard.gp for an example of how to
attach commands.
- Any number of outlier fields may be specified.
- dragon may be restarted and processing continued, with,
for example, a different number of facets.
Example
The following example shows the quickest way to make a wide-field
CLEAN image and display it. Note that this can be more easily done
from the
toolmanager.
include 'dragon.g'
#
# Wait for results before proceeding to the next step
#
dowait:=T
#
# Make an dragon tool
#
drag:=dragon('BF7.MS')
#
# Set the dragon to produce images of cellsize 30 and 2000 by 2000 pixels
#
drag.setimage(nx=2000,ny=2000, cellx='30arcsec',celly='30arcsec', nfacets=4);
#
# Weight the data
#
drag.weight(mode='briggs', pixels=2000);
#
# Make and display a clean image
#
drag.clean(niter=1000, levels='1Jy 0.3Jy 0.1Jy 0.03Jy 0.010Jy', amplitudelevel='0.015Jy',
model='BF7.clean.model', image='BF7.clean.image')
#
dd.image('BF7.clean.image')
#
# Fourier transform the model
#
drag.ft(model='BF7.clean.model')
#
# Plot the visibilities
#
drag.plotvis()
#
# Write out the final MS and close the dragon tool
#
drag.close()
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