Have to go along with Andrew on this one, & am actually suprised to see so many using inverters...unless ofcourse you already had such a set up. DC-DC by far cheaper and more practicle. I to use a measly 18amp hour that keeps me going all night in power saving modes. I also use 2x10watt solar panels off eBay and the system keeps me going night after night
Better still...if your just observing, grab an iPhone & download starmap....will keep you going forever.
Ofcourse depending on how much processing you will be doing; just be mindful that can efect the rate at which you will drain your battery.