I don't use that lens, I have a 300mm F4, 70-200 F2.8 and a Kenko/Miltol 200mm F4. I use the ZWO ASI183MM Pro with the ZWO Nikon adapter. Primes tend to work better (if you can afford them). I'm curious, why stop down that far? F4 should be more than enough to get a decent flat field. I never go beyond that. This was a test shot using the Kenko, 30 exposures at 180s each through an Astronomik NB filter. A lens correction in Lightroom or Photoshop would help, but that image seems a bit worse than normal in the corners, which leads me to believe the spacing isn't quite as accurate as it should be. The image blow is barely cropped, LR corrected for the 200mm lens used. Also, since the majority of the stars in the top right are elongated, it looks like youi have some image tilt in there as well.
Thank you Bob for the sharing.
About spacing I'm sure it's correct. 6.5 of ASI camera backfocus + 20 filter wheel + 20 of zwo adapter give the 46,5mm for Nikon lenses.
The tilt you mentioned make me thinking.
Yes, the image is cropped due to a) the black borders of stacking due to different night work and b) to eliminate the worst of the stars.
Looking at one original uncropped frame, one of the corners is more evident in elongation thus, your assumption may be correct. I must check carefully for tilting. aside a try and error way, there is any "real time" way to accomplish this check? I may need to verify maybe in ccdinspector software.
The f5,6 was to use only the centre of the lenses as it seems the best based on review. But surely, looking at result, after careful tilt check, I will try also with what you suggest.
Thank you again.