dragon - Tool

Postscript file available Tool for wide-field synthesis imaging

include "dragon.g"

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()

Constructors

`dragon`	`Construct an dragon tool`

Functions

`advise`	`Advise (and optionally use) parameter values`
`done`	`Terminate the dragon process`
`filter`	`Apply filtering`
`image`	`Clean and self-calibrate`
`setimage`	`Set the image parameters for subsequent processing`
`setoutlier`	`Set the image parameters for an outlier field`
`uvrange`	`Apply uvrange to the visibility weights`
`weight`	`Apply weighting to the visibility weights`

Next: dragon - Constructor Up: imager - Module Previous: imagermultiscale - Function Contents Index

	`Package`	`synthesis`
	`Module`	`imager`