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Excellent discussion, Steven, and once again a stunning image.
Not thinking about your image specifically, but just thinking about general mechanisms, what might create a very wide artificial halo around any object? I can think of four. There might be others.
The first is actual physical smearing of light from the main galaxy by the optical path. Dust in the air, fingerprints on the optics, reflections off shiny bits. Perhaps only a thousandth part of the light from the galaxy needs to get spread to create a detectable false halo. You could perhaps check this experimentally by photographing Rigel, and seeing what the background looks like when very strongly stretched.
The second, as your ESO colleague mentioned, is any kind of low pass filtering operation (smoothing). For example, a wavelet filter with too many layers. To spread say 400 pixels, you'd have to use a wavelet filter with 8 or 9 layers. You could check by applying the same filtering to the Rigel shot, or by reprocessing with fewer wavelet layers.
The third general mechanism (unlikely to apply in the current case) is superb but not quite perfect flats. Residual directionality in the test light used to produce the flat (or not having exactly the same optical path) can produce quite complex gradients. We're only looking at an error of a fraction of one percent Rephotographing with the object off to one side would resolve that issue.
The final one I can think of is the use of a gradient removal tool. This can effortlessly produce a DARK halo around a galaxy. Perhaps a complicated gradient removal tool can produce a dark halo further out, causing an apparent light halo inside it. The only way I can think of to test this is to do the processing without a gradient removal tool.
Once again, a superb photo, and I don't believe I'm directly addressing your image, but this kind of 'systems engineering' approach might be useful to anyone exploring undocumented super-faint, extensive haloes.
Very best,
Mike
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