If the aurora is crackin and I want to shoot NOW, I just set my camera for ISO 400, f2.8, 30 seconds and fire away. I WILL get a reasonable shot of some kind - stars and all. If you go beyond a minute you'll definitely get star trails, which just look like smudges until you have 20 or thirty minutes. Shooting RAW really helps as you can crank up the contrast, darken the background and get the most out of the image. If you shoot jpg it's ok but you won't be able to get the best possible tweak from the image.
Here is a shot from at 16mm that I cranked on. Humorously you can see a galaxy 1/4 of the way in the frame from the left side, right about in the middle, vertically. Somebody pointed that out to me. cracked me up. The coloration, of course, is the aurora. http://www.pbase.com/santa/image/34967253
Here is a stacked image from 9 exposures. It was done with the TC timer at 2 min each. Unfortunately you get a one second break between exposures with this timer solution. I stacked them in Photoshop. I could have made it brighter but I wanted the deep night appearance. I like it.
http://www.pbase.com/santa/image/37601424
You can see EXIF data for most of my aurora shots at http://www.pbase.com/santa/aurora . You'll see that the majority are 15-30 seconds ASA 400 f2.8. This is one of the biggest reasons to own a 16-35 f2.8, or some reasonably fast wide angle. Otherwise you are looking at 1 or 2 minute exposures.
Mt. McKinley is the peak in the distance here. I stayed out all night that night and the lights were flashing, popping, shimmering, moving all around. I was in awe - and I see this stuff pretty regularly. :)
http://www.pbase.com/santa/image/33824252
You don't need image stacking to merely shoot stars. If you want long trails with a 3 hour exposure that would be too bright, then you can shoot multiple images and stack them to get long trails, but to get a shot of pin point stars no stacking is needed.
Obviously a good tripod and a remote timer is helpful. My MKII won't shoot beyond 30sec without a timer. If I forget it, I vary the ISO instead. If I didn't have one I'd use the 2 sec. timer on the camera.
snip Bud: the night-time sky is my nemesis currently - what's the trick to actually get stars to show up? Image stacking is all I've heard of to date. Our sky is so dark and chock full of them, you'd think it would be easy.
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