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Thanks for all the input. I’ve been pondering this a bit.
From what has been said, its clear that cold air and a warm mirror will degrade the images, but is not a dew hazard – cooling your mirror for an hour or so at the start of the night and possibly using a gentle dew heater later, seems to sort that issue.
The question then, is what to do when the air is warm and the mirror cold?
A frequently seen example would be when someone who has been outside on a cold day wearing glasses goes into a warm house. The glasses quickly dew up. At that point many people wipe the water from the glass surface but, if you wait, the dew clears unaided as the temperature of the surface of the glass approaches that of the air and evaporation, caused by air movement, takes it away. This suggests that, for my telescope, the question is how to minimise the condensation and speed up the evaporation?
Warming the surface of the mirror by attaching heaters isn’t possible as they block the light and I am not aware of cheap tube mountable infra-red LEDs that could do the job.
Warming the back of the mirror can be done and will have an effect, but glass is a poor thermal conductor and a strap dew heater (or two) on the outside of the tube wall near the mirror may be the best we can do.
Air movement will encourage the evaporation of the dew, so a fan is also needed. If the fan is off, the air above the mirror will stagnate and the condensation will persist for hours. If the fan is on the mirror will reach equilibrium faster and, while the dew will form faster, it will also evaporate faster. So, the question becomes: what is better for the mirror short lived dew ups or long lived dew ups?
To my mind, water droplets sitting on the mirror for hours is very bad news so, when the telescope is not in active use, I will try running my dew heater on a higher setting than I normally use, plus I will run my mirror cell fan.
I hope this makes some sort of sense – please let me know if I am being exceptionally dumb as I may have missed something. I will report back here in a week or two. I wil inspect the tube every morning for a week or two.
One aside: I have used a Pulsar, a SkyShed POD and a roll off wooden roof and never opened up the roll off roof and discovered the telescope significantly dewed up. I am not entirely clear why that would be. Bigger air volume – insulated walls/ceiling/floor?