i use iso 800 and exposures of 6 mins , this has been brought about by trial and error but it works for me. brighter objects less of course,
i note that very few dslr exposure runs go beyond about an hour, that might be a limiting factor, diminishing returns perhaps.
i think the really faint objects are in the realm of cooled ccd cameras that can do longer low noise exposures , you can do them with a dslr ... but not a superb job, for the bright objects i reckon dslr is just fine.
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