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Do you mean a square'ish halo not quite centered? The 294 is more or less known for that but I Have found that with lots of subs you need to stretch the data pretty hard to see it.
Are you using flat frames? That should sort vignetting, but on the 294 the flats need to be taken at an exposure time of around 5 seconds or more of they will cause problems. The sensor in the 294 is a SoC (System on a Chip) design and below something like 4 seconds (I have not found out the exact time) the exposure is controlled by the SoC, and above the threshold, it is done by the external electronics made by ZWO. That means different electronics are active and the noise profile changes.
It is like the difference between a timed exposure and bulb on a DSLR.
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