if you keep searching, there are a number of other older threads on this topic. here's an old test i did that is still relevant for canon cameras:
http://philhart.com/content/astropho...xposure-length
i'm convinced that higher ISOs are the best option (for the canon cameras i have used) until your sub-exposures are long enough that you are burning out bright areas, when you can start to dial down the ISO a little. for short subs, ISO1600 is best, but you can hardly tell any difference with 800 too. Higher than ISO1600 makes no difference as it is only a software gain rather than anything in hardware so signal to noise stays the same.
this
pic was taken with 3 minute ISO1600 subs. if you know of a better SLR shot of this region, then I want to see it!
