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Focal length = image scale.
You need the longer focal length without significantly increasing your focal ratio, otherwise, as Paul said, you need longer exposure times for the same amount of light input.
I want to image at around 1000mm focal length as my next DSI scope, and for me, that will be an 8" f/4 newt. So it will be faster than the f/6 480mm ED80 (with reducer) i'm imaging with now.
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