Since m4/3, and particularly Olympus, are a part of the discussion, I will add a couple of comments. I have the Olympus E-M1 and several Olympus lenses, and I love it. However, there is one thing about it that has disappointed me, and given that night-sky photography is one of your interests, it might disappoint you too. At progressively longer exposure times over about 30 seconds, the noise from the E-M1 sensor becomes increasingly extreme, even at base ISO, to the point that it can't reasonably be addressed in post processing. This is despite the fact that at shorter exposure times (< 30 Sec), the noise is quite reasonable and correctable in post, even at high ISO values. If you turn on the camera's option for Long Exposure Noise Reduction, then the long exposure results are much better and quite usable. However, LENR achieves this result by making a second "black frame" exposure equal to the time of your original exposure, during which time the camera is busy and you can't make another shot. So for example a 4 minute exposure ties up your camera for 8 minutes. I think this is acceptable if you only take occasional long exposures, but if you do it often, it becomes very frustrating.
If you search for E-M1 long exposure noise, you will find more information in user forums. I don't know if the E-M5 has the same issue, but I suspect it might.