As Graham mentioned, you need to be in aerial mode to keep the back from taking a dark frame if you want to stack. If you want to take a single long exposure (not the best for star trails) then you will want the dark frame as you will need it to help reduce the noise (at least in a 30min to 1 hour exposure.
Where the Sony should do well is in a series of stacked exposures, with long noise off, however based on what I have seen from the Sony A7rII, (which is showing way too much stuck pixel noise in anything longer than 30 seconds with long noise off), I don't know what to expect from the 50MP Sony. I never tried to shoot the IQ150 I had briefly for anything longer than 30 seconds and that was in a normal setup.
If you want to stack, with Medium format, then the biggest issues is total lack of an intervalometer for the camera. Currently the XF doesn't even have a cable remote (last time I checked and another huge oversight by Phase One, although there are pinouts for one, just can't use the current Mamiya release). Even if you could the current Mamiya release is only a trigger and not a intervalometer. You really need this otherwise you have stand there and manually time the exposures and that is just not a great solution for night work at least.
My article and others will point out that if you want to use the moon to help illuminate your foreground and give a more pleasing look to the night sky, you need to stack as a single long exposure in these conditions will over illuminate the sky and thus only the brightest stars will leave trails. (exposures of 30min to 1 hour).
The software to close the gaps, is "startracer", PC only but does a great job. I don't know if it will work on a Phase One file as it was designed around 35mm full frame and APS-C cameras.
Paul
