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The best way to minimize noise in these shots is to use multiple shorter exposures that equal one longer exposure, and then combine the images via "stacking." Since noise is random but image objects are fixed, the object pixels will multiply more times than the noise pixels will.
Brian--you're absolutely right. "Stacking" often times consists of hundreds of images (or frames, in the case of webcams used for afocal astrophotography). I should have clarified before by stating that the long exposures were actually "cumulative" exposures, where you build up "signal" and algorithmically remove the random noise in post.
That being said, there still are "sweet spots," where the best ISO chosen may not be the maximum ISO of the body. And some of that is dependent on the viewing location, as background light pollution will vary from site to site.
For anyone who has tried their hand at astrophotography, you know how frustrating it can be. Mother nature (viewing) trumps everything. It doesn't matter if you have the fastest and sharpest glass, terapixels of resolution, the fastest image processor, or the most stable and accurate tracking mount. If you don't have clear and dark skies, you won't easily get the final image that you want. It's a very time-consuming craft!