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Collimation of those scope is pretty finicky. You need to spend a fair bit of time at it to get it right. That said this is pretty close to collimation but some extra tweeks are still required but not much.
The 8" scope cools really quickly and would be ready after 1.5 hours. The mirror is a thin conical and does not exhibit tube currents after 1 hour. The tube is carbon fibre and will cool within minutes of exposure to the night air. It is thermally stable and its expansion and contraction is minimal compared to metal tubes.
Once you get it collimated with the secondary you need to take another shot and see how that presents. Be methodical with your elimination of what is going on. If you are convinced the seeing is ok on any given night, then you need to eliminate secondary collimation. If that is ok, then primary collimation needs to be looked at. I doubt this is in issue based on the image.
I am thinking it is just a little out of collimation and some bad seeing. Feel free to post a focused shot of the entire field of your camera over 3 minutes (ensure guiding is working). That will help us more to help you.
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