I am not sure this method is known worldwide or not. I saw a blog article by a Japanese guy recently and found it's good, then I wanted to share it to the world.
The dew fall on the telescope or anything occurs when the thing is colder than the air. The water contained in the air as vapor condenses as water on the cold surface due to the reduction of the vapor capacity of the air by the temperature drop.
Thus, the conventional counter measure is heating up the thing a bit.
By the way, why the thing becomes colder than the air? It is the point! It is by the radiation cooling, the heat radiation from the thing out into the far away space.
Ultimately, the temperature of the outer space is said to be only 3 Kelvin (back ground radiation). Practically, however, it should be much higher by the green house effect of the air, but can be much lower than the near by air.
If you shield the thermal radiation from the scope, especially from the neighborhood of the lenses and mirrors, the zone is protected from the excess cooling by radiation and kept at similar temperature with the near by air. It will prevent the dew fall.
How to shield the radiation? Wrap the telescope, near the front lens, primary and secondary mirrors etc. by a thin aluminum coated picnic mat.
Shiny metal surfaces have very low radiation efficiency. On the other hand, water, ceramics or organic material (plastics, paints) have big values, that means the radiation cooling is strong. Telescope tubes are usually painted. It is actually worse than naked metal in terms of the radiation prevention.
A guy in Japan (↓links below for the original article in Japanese) got this idea recently and reports its really effective. Maybe you need some other measures like blowing dry air onto the lens etc. when dew fall is intense, though.
And, a nice side effect of this is that it will stabilize the air inside the tube. The guy reports also the effect on the stability of the image, e.g. planets.
The radiation is actually an important factor making the turbulence inside tubes. It was thought that the inside turbulence affects for a while after you take out the telescope from your room to the outside cold. But, actually, the radiation cooling continues even after the telescope is cooled to the outside air temperature. It over-cools the tube, and makes continuous convection inside the tube.
The conventional heating method for dew protection makes air convection around the optical parts and it can be harmful when you magnify the image a lot, like planet photos. This radiation shield wrapping method is free from such things.
Here's my scopes with the wrapping:-)
You may doubt that only the outward wrapping has an effect. The secondary mirror inside exchanges heat by radiation with the tube wall, and this tube wall exchanges the heat with the outer world. So, stopping the heat removal from the outer wall has effects on the secondary mirror temperature.
You will see this method works and the dew fall reduces appreciably.
Here's the Japanese blog articles that I saw:
The guy says he got the idea based on the teaching from a guru of drying technology from India :-)
