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with a DSLR I did an experiment, by taking 30 minute subs of the Helix, and also some 10 minute ones. adding two 30 minute subs returned a noisier result than six 10 minute subs, no doubt the un cooled sensor was picking up too much dak current over 30 minutes. For cooled cameras that wouyldnt apply and one cound go longer. It also does depend on the dynamic range of the object. eg M83 has a bright core that might saturate in too long an exposure
Scott
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