if of any interest Peter, modelling suggests a sub length for your system under dark sky of about 20 minutes. How does that compare with your experience?
I am very wary of the concept of diminishing returns for 2 reasons:
1. you restrict your exposure options without any consideration of the system design - scary
2. planetary imagers happily use 10,000 frames and up - each frame helps, even though any individual one could be left out with no practical effect
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