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qimager.makeimage - Function

	Package	synthesis
	Module	qimager
	Tool	qimager

Calculate images by gridding, etc.

Synopsis
makeimage(type, image, compleximage, async)

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

model

Make the dirty image from the MODEL_DATA column

corrected

Make the dirty image from the CORRECTED_DATA column (default)

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

Note the full qimager 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 qimager 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		Type of output image
		Allowed:	String
image		Name of output image
		Allowed:	String
compleximage		Name of output complex image
		Allowed:	String
async		Run asynchronously in the background
		Allowed:	Bool
		Default:	!dowait

Returns
Bool

Example

imgr.ft(model='3C273XC1.model', complist='3C273XC1.complist');
imgr.image(type='residual', image='3C273XC1.residual')
imgr.image(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.