Was this scope bought new or used? If used there is no telling what the previous owner did to it. If bought new, you are probably out of warranty but it would not hurt to contact the vendor.
I am wondering if the size and or placement of the secondary mirror could cause that. My club has an 18 inch imaging newt that is used visually for outreach every Saturday night. It has an oversized secondary mirror. If I place a 35 mm eyepiece in it, the center obstruction appears as a dark circle in the center of the eyepiece at twilight but is not noticeable when it gets dark. I have tried imaging with that newt a couple of times but have not seen any issues with it that matches yours.
Scope was new to me. And I hate to complain to the manufacturer when I don't have any basis yet on what the cause is. I can't say it's any part of the scope. Could be my coma corrector, camera, or mods I've made to the scope. Others on these forums have the same scope and have not had these issues.
Can you share a subset of your data? Flats, biases/flat darks, darks, and lights?
First, I think ABE is hurting you more than helping you. ABE alone can CAUSE these kinds of issues. I would try DBE only. That said, I think this could also be the result of internal reflection issues within your scope and imaging train, possible issues with filters if you are using any, or other internal scope/train issues.
LP filters are often the cause of differences between flat field and light field. Filters often use interference effects to perform their filtration, and these effects can be highly dependent on the angle of incidence of the light. If your flats are from a flat fielder at the aperture, or off a wall or TV screen or something like that, the angles can be significantly different than from the sky/space. Pointing your scope at a flat blue sky, and exposing so that each/all channels fall within the middle of the ADU range could help, since light from a flat blue sky (and, note, do NOT use any kind of diffuser!) should have the same collimation as light pollution at night, and should avoid issues that result from different angles of light in the flats vs. lights.
Here is some sample data including bias, dark, flat, light, and calibrated light (https://drive.google...9fg95jXSuFJF6On). The halo is very clear after calibration, or with a quick background extraction before calibration. Taken with a QSI683 @ -15C, Astrodon E-series luminance filter. It's a dim low-signal target, but one I'm currently working on.
I will try some sky flats this weekend (Friday is supposed to be clear) and see how they compare. I think I understand about the light collimation, and that makes sense. Though taking sky flats sounds like a general PITA.
I must say those lights look familiar.
Ive had VERY similar ones, and I have a very similar setup - 16" f4 and 12" f5 newtonians with a coma corrector.
I think its down to stray light entering the draw tube from unwanted angles. Fitting a dewshield certainly seemed to help me a lot, flocking may have done too. Also flocking upper parts of the secondary mirror holder. Anything that blocks stray light.
Get flocking, and make sure you have a dew shield on the front of the scope. I know you say you have, but maybe it wasnt long enough? Not itself well flocked?
One thing that did help me for sure was using flats made from lights. For example, at the end of the night, leaving the scope taking subs while not tracking, and making a flat using kappa-sigma rejection of all of those 'trailed' subs, to remove the stars. These worked much better than my standard flats, presumably because they better keep a record of the stray light entering the tube.
An example of mine, maybe not the best:
On this night I had moonlight shining into the scope which made it particularly bad. Do you have lights around?
I have some flocking and have hit random parts of the scope. I also hit virtually everything with flat black paint earlier this week. A full flocking of the tube could be on my list if needed. Adding the dew shield didn't make a difference in the artifact, but it does add to the wind sail effect, so I don't want to use it if I don't have to.
But yeah, the sky flats definitely sounds like a possible solution.
I do not have any local lights currently, the scope is in the middle of 10-acre square with trees all around except where my house is. No exterior house lights on, and many of the images are taken with no lights in the house on at all. But some of the images were taken with a lot of local light from my old house. The amount of local light doesn't make much of a difference. But the brighter the overall light (more light pollution, partial moon) does make the ring show up more.
Edited by pbkoden, 18 March 2019 - 06:09 PM.