Always a pleasure to be reminded of that incident Nick. Many thanks. Another round of counselling coming up! 🙁
I now have two – 60w and 120w – thermal tubes located at the mirror end of the 51cm. They take it in turns to keep the primary cosy, and it seems to work OK. The one that caught fire 18 years ago was kept inside the (horizontal) tube of a 40cm. It worked like a dream for many years – until it didn’t.
I do get some condensation from the plastic roof of the observatory, but I don’t worry about it. Just cover the scope with a tarp! I’ve been using the same ‘hinged roof’ observatory/shed for 31 years now, and it’s never been an issue. My previous observatory was of the same design which I used for over 10 years and I didn’t worry about condensation in that either!