There are a number of tricky things to astrophotography:
- Exposure too long (> 8 seconds with a 28mm lens) and the stars look blurred from their own movement.
- Raising ISO to cut exposure time can add too much noise.
- Background light pollution drops the contrast of the sky, making it difficult to expose much longer than 10 minutes (even in the mountains as here). Those shots ones sees from New Zealand just do not exist in my neck of the woods.
- Weird camera problems with long exposure noise reduction (a few shots have the stars wiped out in corner with the Nikon D810).
The Zeiss Otus 28mm f/1.4 APO-Distagon is about as good as it gets for astrophotography in terms of lens speed and control of aberrations.