It isn't about step size it is rather about consistency. If you start small steps and than step increased to a very large value I suspect a error and filter. It works even in non-tracking mode with very large steps.
But if step size increased more than previous (ie 36 pixels after 4.5) - it means something wrong. I suggest check on real setup.
Another option of step starts to be significant (but this is ok) you press pause and resume - I reset limit on the step.
I fully agree with you that the tracking is not consistent. Looking into the subs one by one like a slide show, I can see the target oscillate wildly along the RA line with net shift in RA/DEC also. This is a result of a huge periodic error + poor polar alignment. Currently I can't do too much about it as the hardware is rather locked.
I take your advice to try to simulate pause/resume to see the effect, and more importantly to learn your logic on how to reset the limits to avoid too many fail-to-stack. I extract out the major lines from the debug file and paste below. My observation is that whenever I do a pause/resume action, the next frame for certain can stack. The lead me to make a short trial of having pause/resume for every sub, certainly this make every sub stackable. But this is not practical. I also try to see if the limit and avg_step will be updated by these pause/resume actions. I note that the limit do change a bit but the avg_step not. But suddenly after 13 stacks (seen from the debug) the avg_step is updated and the limit increase (highlighted in bold). So what trigger this update in avg_step and how can I bring in this update earlier? Of course the root cause of the problem still lies with poor tracking, this I understand. Following is the extract from debug:
2024-09-06 09:07:10; stacker, info: Real time video frame 2 arrived (video_stream.h:119)
2024-09-06 09:07:10; stacked, info: So far stacked 1
2024-09-06 09:07:34; stacker, info: Real time video frame 3 arrived (video_stream.h:119)
2024-09-06 09:07:34; stacker, info: Registration at 1:[4, 2]
2024-09-06 09:07:34; stacked, info: So far stacked 2
2024-09-06 09:08:05; stacker, info: Real time video frame 4 arrived (video_stream.h:119)
2024-09-06 09:08:05; stacker, info: Registration at 2:[13, 3]
2024-09-06 09:08:05; stacker, info: Step size 9.06 from (4.0,2.0) to (13.0,3.0) limit = 8.9 avg_step= 4.5
2024-09-06 09:08:05; stacker, info: Failed to stack frame (stacker_processor.cpp:120)
2024-09-06 09:08:31; stacker, info: Real time video frame 5 arrived (video_stream.h:119)
2024-09-06 09:08:31; stacker, info: Registration at 2:[22, 5]
2024-09-06 09:08:31; stacker, info: Step size 18.25 from (4.0,2.0) to (22.0,5.0) limit = 13.4 avg_step= 4.5
2024-09-06 09:08:31; stacker, info: Failed to stack frame (stacker_processor.cpp:120)
pause + resume
2024-09-06 09:09:01; stacker, info: Real time video frame 6 arrived (video_stream.h:119)
2024-09-06 09:09:01; stacker, info: Registration at 2:[32, 6]
2024-09-06 09:09:01; stacked, info: So far stacked 3
2024-09-06 09:09:22; stacker, info: Real time video frame 7 arrived (video_stream.h:119)
2024-09-06 09:09:22; stacker, info: Registration at 3:[40, 7]
2024-09-06 09:09:22; stacker, info: Step size 36.35 from (4.0,2.0) to (40.0,7.0) limit = 15.3 avg_step= 4.5
2024-09-06 09:09:22; stacker, info: Failed to stack frame (stacker_processor.cpp:120)
pause + resume
2024-09-06 09:09:55; stacker, info: Real time video frame 8 arrived (video_stream.h:119)
2024-09-06 09:09:55; stacker, info: Registration at 3:[46, 9]
2024-09-06 09:09:55; stacked, info: So far stacked 4
pause + resume
2024-09-06 09:10:13; stacker, info: Real time video frame 9 arrived (video_stream.h:119)
2024-09-06 09:10:13; stacker, info: Registration at 4:[49, 11]
2024-09-06 09:10:13; stacked, info: So far stacked 5
2024-09-06 09:10:43; stacker, info: Real time video frame 10 arrived (video_stream.h:119)
2024-09-06 09:10:43; stacker, info: Registration at 5:[51, 12]
2024-09-06 09:10:43; stacker, info: Step size 48.05 from (4.0,2.0) to (51.0,12.0) limit = 16.7 avg_step= 4.5
2024-09-06 09:10:43; stacker, info: Failed to stack frame (stacker_processor.cpp:120)
pause + resume
2024-09-06 09:11:13; stacker, info: Real time video frame 11 arrived (video_stream.h:119)
2024-09-06 09:11:13; stacker, info: Registration at 5:[50, 14]
2024-09-06 09:11:13; stacked, info: So far stacked 6
pause + resume
2024-09-06 09:11:32; stacker, info: Real time video frame 12 arrived (video_stream.h:119)
2024-09-06 09:11:32; stacker, info: Registration at 6:[51, 15]
2024-09-06 09:11:32; stacked, info: So far stacked 7
pause + resume
2024-09-06 09:12:03; stacker, info: Real time video frame 13 arrived (video_stream.h:119)
2024-09-06 09:12:03; stacker, info: Registration at 7:[47, 17]
2024-09-06 09:12:03; stacked, info: So far stacked 8
pause + resume
2024-09-06 09:12:28; stacker, info: Real time video frame 14 arrived (video_stream.h:119)
2024-09-06 09:12:28; stacker, info: Registration at 8:[32, 17]
2024-09-06 09:12:28; stacked, info: So far stacked 9
pause + resume
2024-09-06 09:12:51; stacker, info: Real time video frame 15 arrived (video_stream.h:119)
2024-09-06 09:12:51; stacker, info: Registration at 9:[17, 18]
2024-09-06 09:12:51; stacked, info: So far stacked 10
2024-09-06 09:13:09; stacker, info: Real time video frame 16 arrived (video_stream.h:119)
2024-09-06 09:13:09; stacker, info: Registration at 10:[6, 19]
2024-09-06 09:13:09; stacker, info: Step size 17.12 from (4.0,2.0) to (6.0,19.0) limit = 17.9 avg_step= 4.5
2024-09-06 09:13:09; stacked, info: So far stacked 11
pause + resume
2024-09-06 09:13:28; stacker, info: Real time video frame 17 arrived (video_stream.h:119)
2024-09-06 09:13:28; stacker, info: Registration at 11:[2, 21]
2024-09-06 09:13:28; stacked, info: So far stacked 12
pause + resume
2024-09-06 09:13:49; stacker, info: Real time video frame 18 arrived (video_stream.h:119)
2024-09-06 09:13:49; stacker, info: Registration at 12:[2, 22]
2024-09-06 09:13:49; stacked, info: So far stacked 13
2024-09-06 09:14:04; stacker, info: Real time video frame 19 arrived (video_stream.h:119)
2024-09-06 09:14:04; stacker, info: Registration at 13:[7, 24]
2024-09-06 09:14:04; stacker, info: Step size 5.10 from (6.0,19.0) to (7.0,24.0) limit = 25.0 avg_step= 12.5
2024-09-06 09:14:04; stacked, info: So far stacked 14
2024-09-06 09:14:42; stacker, info: Real time video frame 20 arrived (video_stream.h:119)
2024-09-06 09:14:43; stacker, info: Registration at 14:[14, 25]
2024-09-06 09:14:43; stacker, info: Step size 7.07 from (7.0,24.0) to (14.0,25.0) limit = 21.3 avg_step= 10.6
2024-09-06 09:14:43; stacked, info: So far stacked 15
2024-09-06 09:15:21; stacker, info: Real time video frame 21 arrived (video_stream.h:119)
2024-09-06 09:15:21; stacker, info: Registration at 15:[26, 26]
2024-09-06 09:15:21; stacker, info: Step size 12.04 from (14.0,25.0) to (26.0,26.0) limit = 19.7 avg_step= 9.9
2024-09-06 09:15:21; stacked, info: So far stacked 16