Nikon's new D5 offers up to 3,280,000 ISO both for stills and 4K UHD video.
Although expensive at £5,000 it should mean that this technology drifts down to more affordable cameras.
20.8 m pixel full frame.

Things are getting interesting.

In the recent BBC T.V. astro programmes the cameras were showing fulls sets of stars in real time down to perhaps 6th mag. The views looked really realistic in wide angle shots.

Real time Aurorae video is also now common.

Regards, David

The companion D500 is one third the price of the D5 and shares many features.
It goes up to 1.6m ISO, but the D500 video goes for 30 minutes instead of 3 minutes with the D5, which cuts off to prevent heat damage from the processor.

So the D500 might be a good seller, especially when the price reduces with time.

Regards, David