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I personally try not to get stuck on f/ratio. You're always going to be sacrificing resolution (image scale) imaging speed unless you're willing to spend the money buying large fast scope.
Even if you need to increase it to f/20 and its corrected, go for it! It'll take longer to get your SNR up but you'll get what you want. Most of my research over the years has been done on an f/10 with no issues and I've even considered pushing that to ~f/40 for planetary imaging.
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