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Optimum ISO seems to be a point of debate. Often quoted that ISO1600 is a ceiling. I got my best DSLR image at ISO3200 in winter.
ISO is really just the gain of the amplifier circuit at each pixel. But I did read on another forum that the way it works its not just as simple as amping everything up including the noise.
I think the short answer would be it would depend on your camera.
5D mk ii may be best at ISO1600 and other cameras ISO800. 5D mark iii or Nikon D800 which have superior high ISO low noise performance may be better at ISO3200 or 6400. Perhaps even higher for the mark iii in jpeg. Both cameras shoot up to ISO12800 cleanly and 5D mk iii even higher in jpeg only.
Ideal exposure length would be measured by amount of histogram filled. Some say 1/3rd histogram, another said 3/4. I am not sure of this point.
I would say the point where stars still retain their colour and not beyond.
You may need to find that point by trial and error.
At least in dark skies you don't have to worry so much about sky glow.
I am about to shoot some DSLR astroimaging myself once I get a Nikon D800E but that will be a while as they aren't in the streets yet. Mainly Milky Way widefield mosaics and time lapse but also some deep sky objects to see how 36mp looks on a fast APO with perfect optics.
Greg.
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