Thank you all for the replies! Lot of useful info. This is getting closer, but let me further clarify my issue: say I would like to take a series of 30sec shots. In previous Nikons, because of mirror movement and other processing, there is a small delay between each 30s shot (2-3 sec).
When photographing startrails or cityscapes during night, this shows up: there are ugly gaps every 30 seconds.
I'm asking, whether the new D850 with its all-electronic shutter is there a straightforward way to set up the shooting of such timed sequences: e.g. 30 sec exposures - but every exposure begins where the previous ended, with no blackout.
EDIT: of course, these timed shots only make sense if this sequence can be kept up for hours...
EDIT 2: the smaller exposures cannot be substituted with one, lengthy exposure: 100x30s is not the same as 1x3000s. Reasons are "astronomical"