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imager operates on a specified MeasurementSet to produce any of a range of different types of image: dirty, point spread function, clean, residual, etc. A MeasurementSet is the holder for measurements from a telescope. It is simply an CASA Table obeying certain conventions as to required and optional contents. The intention is that it should contain all the information needed to reduce synthesis and single dish observations (see CASA Note 191). A UVFITS file can be converted to a MeasurementSet using the ?? tool function (a constructor of the ms tool.
imager adds some extra columns to the MeasurementSet to store results of processing. The following columns in the MS are particularly important:
- The original observed visibilities are in a column called DATA. These are not altered by any processing in CASA.
- During a calibration process, as carried out by e.g. calibrater, the visibilities may be corrected for calibration effects. This corrected visibilities are stored in a column CORRECTED_DATA which is created on demand by calibrater and imager. In creating the CORRECTED_DATA column, imager will only correct for parallactic angle rotation. This can be controlled using the ?? tool function. All imaging performed by imager is from the CORRECTED_DATA column (apart from the tool function makeimage which can also make dirty images from the other visibility columns).
- During various phases of processing, the visibilities as predicted from some model are required. These model visibilities are stored in a column MODEL_DATA. These are used by the calibrater tool for calibration.
- Weighting of data (including natural, uniform and Briggs weighting, and tapering) is accomplished by setting the column IMAGING_WEIGHT appropriately.
imager can handle an initial model in a number of forms: as an image, as a list of images, as a ??, or as some combination. Fitting of componentmodels is planned but is not currently supported.
imager uses a number of scratch files. Following CASA practice, these are placed in the directories specified in the aipsrc variable user.directories.work. Those disks that possess sufficient free disk space are chosen in sequence. So to spread your scratch files over two disks each of which has a directory tcornwel/tmp do e.g.
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