How would I fix this?

[ET_PhoneHome]

I have this image of Cygnus, and as you can see, there is a dark arc-shaped section on the right side. It's actually on the left side, too, but I cropped it out. What causes this,and how can I get rid of it?

[admin]

This can only be properly diagnosed by looking at the full res image (as well as knowing the settings you used in Wipe), but it whenever you see Wipe backing off like this, there is almost always a "dark anomaly" nearby; e.g. a bunch of pixels darker than the real signal's background.

Usually these are stacking artefacts or dust donuts. Zoom into the corners and see if you can see them. It is usually easily remedied by cropping away any dark anomalies before running Wipe. Alternatively you can mask out those areas if you really cannot (or do not want to crop those areas), so that Wipe wil not sample these areas for background signal to model.

See also here for what is happening and some other textbook examples of dark anomalies.

It may also be a an imperfection in your flats (e.g. they are under or over compensating for the uneven lighting), but this tends to be a much rarer occurrence. The Vignetting preset (which increases corner aggressiveness) may help in that case.

Hope this helps!