sharpeness of stars on Nikon 14-24mm 2.8 at infinity

just got my new Nikon 14-24mm 2.8 ready for going to Norway in 5 weeks and went outside to test its focus on infinity (manual focus) as I will be photographing the stars and northern lights there and tonight was the first starry night since I got it.

I started photographing everything on infinity but then I checked focus as much as i could in camera out in the field but optimum sharpness of stars didn't seem to be at full infinity, in fact i did some shots with the dial turned gradually further from infinity and it seemed to get sharper a few mm off from infinity then got softer again

is this normal?

or should i get it checked out?

Mike

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www.mikecurryphotography.com