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I have done some long exposure tracked images at night with an A7 which is probably relevant enough for you.
After taking about 10 lights (and accidentally 10 in camera darks) over the span of an hour, I then took 10 darks for 30 minutes in the trunk of my car on the way home, and lastly 10 quick bias frames in the driveway on a fully charged battery in 70 degree F weather. I turned on electronic first curtain.
At that point battery life was 37% for a little over 1.5 hours of total exposure time, verified by my image file date/time info on my computer. So I would think in warm weather, you could probably squeak by on a good fully charged battery for about 2 hours. In cold weather, probably not nearly as good. You may want to look into the ac/dc power supply for the A7r or change out the batteries really quickly.
I forgot to turn off long exposure noise reduction so each light was followed by an equal exposure for the in camera dark frame. Hence, I could have had 20 lights instead. I have since turned off the long exposure reduction feature since I stack in post. This is probably even more important for star trails to avoid gaps in the trails. I don't know how long the exposure has to be before it starts taking in camera darks.
I have a third party wired intervalometer that works fine for any exposure length I set. I imagine any wired release would work
I never use an optical viewfinder for focusing. With Canon, I use magnified live view on a bright star. With the A7, LCD screen 'live view' magnified manual focusing is great as long as you turn off the peaking feature. Just be sure to tape down the focus ring carefully. You may want to set up everything at home and save your settings as a custom mode so you don't have to fiddle with it in the dark.
I seem to be the only one that goes on and on about dew, but if you are out all night and pass the dew point, your affected images will be ruined. I've been burned once, but never again. Only time I have not passed the dew point was at Arches NP. I have a small dew heater strap that I apply after taping the focus ring securely. I use a cheap jump start battery to power the dew heater controller. My camera strap may literally drip from moisture but my glass stays perfectly clear. A 15" strap should cover you for all your lenses.
http://www.thousandoaksoptical.com/dew.html
David
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