REMOVING THE EFFECTS OF BRIGHT SOURCES IN THE FIELD

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A bright point source in the field (offset from phase center) can cause ripples across the image plane when Fourier transforming the visibility data. The further away from phase center, the smaller the frequency of the ripples. Extended sources do not impact the field as much because the different phases (from the different positional offsets) average out more quickly across the image. In general, the more beams away you are, the lesser the impact on your image (and the flux of your object at phase center depending on whether it is located in a peak or trough of the ripples caused by the bright nearby source).

For adjacent sources, for example, in the case of a (visual) binary source, one method is to use 'maths' to subtract a source close to the phase center. The task 'msub' can grab a box from the cleaned image, which you can then subtract off. Convert the data back to the u,v plane without that box using 'uvmodel', and then run 'uvfit' or 'uvamp' with the targeted source defined at the phase center using the appropriate offset or center keyword.

Another method is to select the clean components from the u,v data using uvmodel and then subtract from the u,v data the bright, extraneous sources. ...????

The task 'imhist' will give you the image statistics. ????

If your image has a high dynamic range, then a Direct Fourier Transform (DFT) is more precise in removing point sources because you know exactly where the source is in the u,v plane. In contrast, a Fast Fourier Transform (FFT) takes advantage of gridding, reducing your positional accuracy in exchange for computational speed. The error in position means that you may not be fully subtracting your point source and its corresponding effects from the data.

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