I know with my Canon there is a setting accessed through the menu that is supposed to help cut down on noise from long exposures. It works by taking a second "blank" exposure for the same length of time as the first and sandwiching them together.
This can be a hassle sometimes because for every 10 second exposure you take you have to wait 10 more seconds for the camera to record this other blank image and combine them. That turns your 20 minute star field image into a 40 minute experience (time for a blanket and some hot chocolate).
I don't know if the Nikon offers a similar ability or not, but it's something to look into...
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