Hello there Frustrated Martin.
Well, mono, and especially once you start getting fancy like LRGBHa, is where PI's WBPP really shines. But, that cost of entry, especially if you're not going to be using PI for any post processing. Both Freddy and I are doing it though.
I have no experience with APP but it is supposedly an excellent stacker too. Siril may also have the capabilities that are needed here, but I just still find that program thoroughly confounding. If you can figure it out though, it might do the trick.
That grid pattern is a bit interesting, especially since it appears to be warped. Splined by interpolation maybe? In any event, I wonder what the source is?
Since you already have a 2600, you probably already know that its darks and bias, when hyper hyper stretched, will reveal vertical and horizontal banding. But it is at such low levels that normal stretching should never bring it out. I wonder then about your acquisition settings and calibration files. Are you using something like Gain 0 Offset 30 for everything, or maybe that for broadband and Gain 100 Offset 30 (or 50) for Narrowband, with appropriate matching settings for the corresponding darks, flats, and bias? All files taken through the same program, APT? Everything set up correctly in DSS? And by that I mean no reusage of master calibration files, unless you are totally sure you know what you are loading in, and that the digital development process settings are correct for mono (you have to uncheck the top box)?
Moving on to ASTAP, it's a handy stacker, but like DSS has it's own way of doing things and pros/cons to deal with.
First, unlike DSS, you can't just load in Bias (hopefully you are using darks anyway) and have ASTAP know to use it on both sides, lights and flats. You'll have to load files into all four slots. But, bias of course can be loaded in lieu of dark flats, and while not perfect those bias can also be loaded into darks. It will create a master dark, which you can archive and load in by itself in the future (if applicable) to avoid recombination.
I always tended to do things step by step rather than just load and hit go, so I could inspect what was happening.
So you can load your bias into dark flats and hit analyze if you want. Then same with your flats. In the flats tab, you can hit "replace with" and it will combine the bias and use that to calibrate the combined flats, and you will end up with a master. All other files in the flats and dark flats tabs will be disappeared. If you select Classify By Filter you will get masters for each filter, which of course you want.
That actually gets set up in the Stack Method tab. There, make sure that your naming conventions for the filters match between ASTAP and APT (or NINA). You will know you got that right because when you analyze frames that have filters (lights, flats), the filter column will change color to indicate that it is recognized. Also for mono make sure that nothing is checked in the RAW/OSC settings. Lastly in Stack Method, you are going to want to choose Sigma Clip, skip LRGB combine. This will then create multiple light filter masters (albeit not registered, more on that later).
Then you can load your darks, which can be analyzed and/or replaced with master. Darks also (and Flats) have options across the top for Classify by things like date, which permits the session (group) stacking as you mentioned.
Finally you can load your lights. That must be followed by analyze/organize, which is very similar to running pre-registration in DSS. Once you are happy with the subs you want to stack, you'll probably go set your alignment method tab to star aligned, and then you can just hit stack and watch it go.
If all works well, you will end up with a stack for each filter. "Masterlight" in PI lingo. But, they are not star aligned to each other. Unlike with DSS, you cannot select a reference. ASTAP does that for you, based primarily on HFR/scoring, and that's a drawback for mono, as you can't freshly stack everything to a reference.
So, you clear everything out and then load those master stacks into lights. You may have to manually check the boxes. Then run analyze. Then in Stack Method you would choose "calibration and alignment only." Don't worry about calibration since you have already done that (but again make sure you have cleared out the files from the darks and flats tabs). Then run "stack" and it should then star align all the files to whichever stack had the best HFR, and will spit out new ones - the filename will identify the aligned files.
And those you should be able to then load into compose in ST. However, I can't remember if ST 1.9 is able to pull in the exposure times per filter from ASTAP after having to run that separate star alignment (another benefit of WBPP).
Well, now that I've thoroughly confused you, just go play with it. It's "close enough" in UI to DSS that some experimentation will get you moving along in short order. Also, the ASTAP website has pretty decent instructions.
