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In the past I have used a longer dovetail and braced the focuser tube into an extra ring to hold it. That helped with sagging a bit but you still need a solid focuser so it doesn't slip. I used a bathinov and refocused often at the beginning of the night but as it goes on it doesn't change as much. Few rags on the scope also help with dew and keeping the tube temperature closer to ambient. In bad seeing it's very hard to focus accurately though. Not much you can do then.
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