Bengan

OK, my bad. My answer was without thinking

Answer removed

First of all. When you have issues and ask for help with something, it's always helpful if you supply examples (post photos).

Second, any lens-body combination usually needs a AF fine tuning. I'm a bit surprised that you after 28 years haven't experienced this before. Personally, I always start off a new lens with a AF fine tuning/check.

Try to post the photo you're referring to again. I can't see it either

IQ differs a lot between different mirror lenses. I have no experience with the one you're using here. I have used Tamron Adaptall lenses and they definitely perform a lot better than this. There is also a variance between copies and you can find good and bad with all brands. 

The shot here could also suffer from motion blur. It's hard to tell from only one shot.

I don't think that you could clean away the problem here. Some dust doesn't affect the shots.

Bottom line, mirror lenses can be fun but don't expect them to outperform normal lenses. With a good quality 300 mm you should be able to get netter results cropping.

D810, Tamron 24-70@70 mm, 1/125 sec, f13, ISO 450