The embossing effect is from a mismatch between lights and flats, caused by some shift between taking lights and flats. Most often that is by rotating the camera very slightly, but sometimes is caused by sag in the focuser. I see dust rings all over the image and they all have a dark ring on the left edge, which means they are all shifting in the same direction and roughly the same amount. I would think this is focuser sag over an accidental camera knock. You're using a newt, which likely has a low-profile focuser, and a mono camera, presumably with a filter wheel or drawer. A wheel will add a significant amount of weight and the focuser is likely not quite up to the task. An OAG also adds weight, but it's usually not significant enough to cause this amount of sag. You can nail this down (mostly) pretty easily with a few shots across the sky.
The standard configuration of a newtonian imaging system is to put the camera facing the mount to reduce inertial moment arm, so I will assume you are doing this. Start by focusing on Polaris with the camera in a neutral position facing down. Slew to due east at about 45 degrees altitude and take some lights and flats. Slew to the opposite side on the west and take more lights and flats. Calibrate your east lights with the east flats. They should calibrate perfectly. Now calibrate your east lights with your west flats. I expect you will see the embossed rings again and this is a sure sign of sag. Your rings should have a similar pattern of dark on one side and light on the other. If you calibrate your west lights with east flats, you should see the opposite. If do see this, you have nailed down the change to a shifting light cone. Remember, this is a change in the order of microns, so not something you can see with your eye; you can really only see it in the image.
To make it somewhat more complicated, newtonians have this persnickety thing in the optics called a secondary mirror, and these may also sag under shifting gravity. (Just wait till you get to SCTs...) The test for this is to use a collimating laser and swing the scope from east to west and see if the laser returns true to center. The laser is almost weightless in any focuser so won't cause the sag that a large camera and wheel will.
What's the solution? Take flats on whichever side of the sky you're imaging on. The goal is to let the focuser sag in the same direction for both lights and flats. If you image on both sides of the meridian, take flats for both sides. It's a minor pain, but sometimes it's easier (especially
cheaper) than buying a premium focuser like a Moonlite or Feathertouch or Optec.
As for the background extraction, I don't have much to offer in the way of getting GXP to behave better, but I will offer the alternatives built into PI: ABE, DBE, Gradient Correction and Multiscale Gradient Correction processes. ABE and DBE are the older methods that deal with sample placement and model generation. GC and MGC are more recent additions with notably different workflows than ABE/DBE. Try playing with GC first and see if you can get a good result. MGC is a pretty complex addition to the workflow that isn't intuitive or easy to use, but will result in better results if your target is in its coverage area (it's a part of the MARS database project).
The cause could be a simple light leak, which involves just plugging all the holes that light might seep through (be warned, that can be many, many places). To nail this down, you can put a solid cover over the aperture and shine a flashlight into the scope from any possible joint in the imaging train or near the primary mirror. Set your capture program to continually expose in short exposures at high gain and watch the screen as you move the light around the scope. I found a huge leak in my focuser between the draw tube and the main focuser body. There were no baffles to block light, and plenty of space for bearings to contact the tube, hence a recipe for major light leaks. I covered it with felt and haven't had an issue with it since, and I need to make covers for my other focusers that have the same issue (they're all Moonlites).
By the way! It looks like you have a good start to the image. I'm interested to see how the final image turns out because your data looks really low-noise, especially for a low hour count of 3 hours. Good job and keep going!
Edited by italic, 19 January 2025 - 03:18 AM.
