I keep my imaging scope set up in the back yard all of the time, but I don't have an observatory. I keep it under a Telegizmos 365 cover, with heating rods underneath. I check check underneath it around the pier periodically to ensure that the heaters are running and that there is no condensation, at least where I can reach without removing the cover.
My last imaging session was about 4 weeks ago. We have a forecast for clear nights through Wednesday, so tonight I went out and uncovered the scope. The mount was dry, the outside of the OTA was dry, and everything seemed fine.
When I powered up the mount and positioned it parallel to the ground for access to the dust cover, water started running out of the tube vents. When I took the dust cover off, I found that the outside of the corrector was dry and there was water pooled inside of the tube. I positioned it so that there was a vent at the low point to let all of the water drain out, and I have the dew heater running warmer than usual to dry the corrector.
I've kept my system outside like this for about 5 years, and this is the second time I've had this happen. In this case, the OTA is an EdgeHD 8. It happened once before to my C11 (non EdgeHD, without vents - I had to remove the corrector plate that time and dump at least a quart of water out of the OTA). It has never happened with a refractor mounted, and it's never happened with my small RC.
This is not a case of the TG cover leaking. For one thing, I use different covers for the 8" and 14" scopes. Also, I've encountered seriously inclement weather with driving rain and worse in the field - and the covers don't leak a drop.
It has to be condensation that's getting into the tube, but I don't know what conditions are allowing it. As I said, the exposed surfaces under the cover are dry. It's only inside the tube. If I remember right, the time that it happened with my C14 was also in around the end of the year. I wonder if there is something about significant temperature swings (15 to 20 degrees F with high humidity) that's trapping cold air inside the OTA.
Any thoughts on what's actually going on? And even better, any way to prevent it (short of bringing in the OTA)?