I've done some searching and haven't been able to find a consistent answer so I'm hoping someone can help me out. I realize that conditions can be highly variable, but taking the most general settings (clear night, stars visble, no moon) is there a exposure rule for star trails that's similar to the Sunny 16/Looney 11 rules? Or if not, is there an aperture/shutter/ISO combination that works for most occasions? BTW, I'm using a D700. What's the max time that I can leave the shutter open without getting negative results from amp glow? Also, sice the use of Noise Reduction essentially doubles the amount of time needed to take a photo, can I turn Noise Reduction off and then take a lens cap photo using the same exposure settings later on to use for noise subtraction? Example, say I take 4 photos each at f/8, 15min, ISO100, all with NR off. Can I shoot a 5th photo with the lens cap on using the same f/8-15-100 exposure and load it into Capture NX to use as a noise reduction reference pic for the first 4 photos?