It’s In The Post noise removal
When you use high ISO’s to capture a night sky full of stars, or if you are fortunate enough to photograph an aurora, you are likely to get noise in the file, depending on the ISO capability of your camera. The Nikon D3s is probably still the best camera on the market that produces little noise at high ISOs. Even with the high-resolution Nikon D810 that I currently use, the noise is minimal at up to ISO1200. However, I will often use ISO3200 to capture the night sky, so the noise is more apparent. In Lightroom, Luminance Noise Reduction does a great job of reducing noise, though take care if you move the slider past 50, as the fine detail will go soft – use the detail slider to bring it back.
Another very good piece of noise-reduction software is Nik Define 2. The algorithms work very well to reduce noise and retain detail at the same time.