I've been doing some comparisons of my scopes on the planets and Moon now that we can see Jupiter, Saturn & Mars on the same night.
I've set up the following over the last few nights:
Orion 110mm ED f/7 doublet
SkyWatcher 6" Mak-Cass (f/12)
Celestron C8 (f/10)
GSO 10" Dob (f/4.9)
I've aimed to set the scopes up at least an hour before darkness, and vented the C8 & Mak by taking the diagonals out and inverting them as far as the mount would allow.
Nightime temperature change is quite slow where I live (Sydney, Australia) - probably only 1-1.5 degrees C per hour. I don't think this is particualry rapid compared to other parts of the world.
I was surprised how long the scopes took to reach thermal equilibrium, even the refractor, which seemed to take at least 30 minutes to become usable at 150x, and took about an hour to really settle down.
Both the 6" Mak and C8 took a really long time to show their best (over 3 hours), but the image was never as stable as the refractor. The Mak had marginally sharper images than the SCT, but this could be the optics rather than thermally related (and in any case the 6" mirror should reach equilibrium sooner than the 8" one).
The 10" dob (with a small cooling fan) was somewhere in between the refractor and Mak/Cass - after a couple of hours it was pretty good and showed the most detail on Jupiter, Saturn & the Moon - but this is also to be expected given its better theoretical resolution.
I'm curious about how I can improve matters - 2-3 hours is a long time to wait for a scope to be "ready" for planetary observation. The biggest problem is the Mak and SCT. I don't really want to be drilling holes is these (& I don't have the tools) - so what can be done to reduce their cool-down times? The scopes are kept in an unheated garage by the way - but this is still probably several degrees warmer than the outside, even in winter.