Hello everyone, im having a problem with my imaging train. I can not quite pinpoint the issue but im thinking it must be the backfocus. Please see the attached images. They are 60s, 180s and 300s exposures of NGC281 which ive used PI's Aberration Inspector on. There is some trailing in the 300s exposure but thats not part of the problem. Stars may look blocky but thats because the frames arent debayered yet.
The Stars on the sides and corners of the frames are elongated, i believe this is caused by backfocus not being perfect. Without the IDAS NB1 filter, this doesnt happen. This filter is 2mm thick and ive followed the rule of thumb of dividing filter substrate thickness by 3 (so 0.67mm) and adding/removing that number to/from the backfocus with spacers. Ive tried 0.67mm metal sheet i got from a metalworker friend but ive also tried 0.67mm 3D printed spacers.
Heres the problem: Nothing changes with either spacer and without any spacers. Focus was as good as i could get it with a bhatinov mask.
Im using a TS ED80, TS Flat 0.8x reducer corrector, an IDAS NB1 Ha+Oiii filter and a self modified Nikon D3400. This also happened when i had an ASI585mc for testing.
So, what do you guys think causes this? Is this just a limitation of my gear, or is my reducer-corrector faulty?
EDIT: Something important i forgot to add is that i modded my D3400 and did NOT replace the stock filter with anything!