Advice on first efforts

@edwallace Ed, does the halo also show up on the lights when you stretch them a lot? Or perhaps in the darks and/or master dark? I occasionally still use an ASI1600MM as well and have been using it in the part two years and have not seen such a prominent halo.

Yeah, that would be really, really weird vignetting.

@wvreeven Attaching one L frame with 2 different amounts of stretching.

@edwallace OK this clearly shows that the ring is present in the lights. You'll need flats to correct for that.

@wvreeven That confirms my suspicions. We've just begun a run of many cloudy nights here, so I will take flats soon. Thank you for the info. 

@wvreeven I spent most of today, off and on, working on this. I took flats and darkflats to calibrate them. In the end, the artifact is still in the image, but it is greatly reduced. This was my first attempt at using flats. I suspect I need better flats - maybe a better light source (I used a white screen on a tv). Still, it is progress.

Not sure if that would be the issue, but a TV might put out a different spectrum from what you want in a flat (broadband), people sometimes also have issues using a tablet for instance. The ring seems optical though, but may be worth trying a different light source like a sky flat or an actual flat-box.