Splitting and Imaging

Although this is largely a matter of taste, it may be convenient (particularly if self-calibration is to be used) to split the multi-pointing visibility dataset into single pointing ones. Task uvsplit (with no options, and only the multi-pointing visibility dataset as input) will do this function. It will also copy across any calibration tables associated with the input dataset.

In the individual approach, you will run invert many times, once for each pointing (you may have split the multi-pointing visibility dataset into single pointing ones, or you could use selection by source name to select out the appropriate subset of data). Apart from possibly the names of the input and output datasets, the parameters to invert should not be changed between runs.

Even though you are imaging just a single pointing, you will still want to use invert's mosaic mode (options=mosaic). This causes invert to perform its geometry corrections and to create the the images of the different pointings on the same pixel grid. In this way, no interpolation will be needed when the images from the different pointing are finally combined. Consequently the artifacts and problems associated with interpolation can be avoided.

To compute the geometry, however, you must provide a reference position on the sky - the tangent point. The default tangent point is the pointing centre of the data being imaged - this is not appropriate as it will vary from pointing to pointing. You will want a tangent point which is the same for all the pointings. Although it can be any arbitrary point, it is best to make it near the centre of the source being imaged. If there is a point source which dominates the image, you might choose its position as the tangent point to help reduce deconvolution problems. The tangent point is given through the offset keyword, in the format hh:mm:ss,dd:mm:ss (or as decimal hours and degrees).

As an example, consider an LMC observation, where we wish to image field 123 (which has field name lmc_123). Assuming we have a multi-pointing dataset, and wish to use position (RA,DEC)=(4:30,-71:00) as the tangent point. Typical inputs to invert would be:

INVERT
vis=lmc.uv	The input dataset.
select=source(lmc_123)	Select a single pointing.
options=mosaic	Use mosaic mode.
offset=4:30,-71:00	Set position for geometry computation.
map=lmc_123.map	Output image name.
beam=lmc_123.beam	Output beam name.
cell=	Set cell size.
imsize=	Set output image size.