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Next: Self-calibration Up: Imaging, Deconvolution and Self-calibration Previous: Wide-field imaging and Mosaicing

Combining single dish and interferometer images

Sometimes, the lack of total-power data in an interferometric observation (single- or multi-field) can result in image defects on the size scale of the interferometer baselines. For example, in a maximum entropy algorithm with positivity constraint, the ``negative bowl'' will manifest itself in the model brightness distribution by a flat region of near zero emission surrounded by a spurious positive ``splash'' around the bowl; the ``splash'' artifact sometimes has small scale structure.

Currently, the only method available in Imager to combine total power measurements with interferometer data is via a feathering technique.

Since feathering is an intrinsically post-deconvolution method, it cannot help such a case. However, there are two ways to help out the deconvolution algorithms. First, convert the single dish image to the same image grid as the interferometric mosaic by using the Image.regrid function. Then either rescale or deconvolve the image into units of Janskys per pixel. This image can be used as a prior image in maximum entropy or maximum emptiness and will prevent small scale image artifacts due to the lack of total power data. However the prior image is not elevated to the level of a hard constraint, so a subsequent pass of the interferometer image and the single dish image through the Imager.feather function will ensure that the large scale structure in the final image is consistent with the single dish image.

imgr.feather(image='feathered.image',      # Specify the single dish
             highres='casa.vlaonly',       # voltage pattern via
             lowres='casa.sd',             # a table
             usedefaultvp=F, 
             vptable='sd.vptable');

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