I have tried working at this. What I can report is that the same tools Trevor is accessing over multiple commercial apps are basically all available inside Siril for free. Some things don't work as automatically, but it's there. Specifically the background gradient tool works differently, some extra options and steps to check off on. But it's there. The star removal feature will only work on PC with a more modern CPU, the 15 year old AMD CPU in my primary photo editing PC cannot run it. There are missing CPU commands it calls. But the AMD CPU in my laptop that I bought 2.5 years ago runs it fine. So I had to transfer files back-forth between 2 PC's to get the work done. Easily done on a common local network. GIMP did the finishing polish. BTW, GIMP 3.0 release candidate 2 is out there. Release candidate 1 was crap and crashed my computer, but ver2 seems to be OK. Stilll have Gimp 2.10 also installed until I have better confidence.
This is what I accomplished with Siril in 2021 with this odd composition of Orion. Kind of too close at 90mm, but also should have been pointed differently and rotated to be more portrait mode. Really, not terrible job of stretching the background exposure.
That is 60 images stacked, shutter was open 70 seconds each shot, ISO 2000, f/3.2 (stopped down a bit on the Tamron 90mm lens for less vignette)
Another run back at the time where I tried to darken the background more evenly. Lost some nebulas, such as the Horsehead.
I had some hope there was a bit of Barnard's Loop deep in the data. But maybe I needed to use f/2.8 and raise the ISO. I tried to recreate Trevor's magic, and sadly not much to extract there. I did make a mess of the color trying to reduce the too-much green. I intentionally used noise-reduction on the star-field to reduce the density of stars visible.
I brought out the Orion Nebula without blowing out the detail. Horsehead Nebula still hidden. Just no sign of Barnard's Loop. But I don't have a full-spectrum modified camera and H-Alpha light pollution filter either. If I tried to reshoot this, I would substitute my Nikkor 70-200mm f/2.8 lens and try at 70mm (lens was not in my kit then). I would raise to ISO 3200, go with f/2.8, and reduce the shutter time to 45 or 50 seconds. That seems to work well with my star-tracker.
