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For planetary imaging, as long a focal length as your seeing will support. I imaged Jupiter with an 11" SCT at f/40 once. 11200mm focal length.
For deepsky. I've imaged everything from 300mm focal length through to 5600mm and f/ratios from f/2.8 through f/20. What is best? That's a matter of personal taste. Do you like wide fields? Narrow field? Something in the midrange?
Exposure times. Go with the longest you can manage with your mount and sky conditions. Some targets (m42 is a prime example) require long and short subs to properly expose.
At home, I run 5 minute exposures due to sky conditions. Under dark skies I run the longest exposures I can stomach loosing Usually 20~30 minutes. I don't like going longer because should something go wrong you don't want to lose a 1hr exposure.
Total exposure per target, this varies with sky conditions, object brightness, speed of the optical system etc. Anywhere from 1 to 150 hours depending on your goal with the image. The more the better is the general rule. I like to dedicate 5+hrs per image.
There are no real answers to these sorts of questions.
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