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ISO6400 will have significantly lower dynamic range than say ISO1600 and may not offer any improvement in signal/noise ratio at all, so using the higher ISO will just clip your star cores for no other benefit.
It would take some good objective testing to work out the right answer.. I'm willing to bet that five minute subs will show bugger all difference between working at 1600/3200/6400, except for the reduced dynamic range which may or may not be an issue depending on f-ratio and subject etc.
Phil
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