How to fight the dew from the top? Start a fan club.
Will it work? Maybe to much vibration? Will find out soon.
Heatwaves in the tube? Is "bad seeing" not better than "no seeing" at all?
It is a small 60mm fan right behind the secondary mirror. I plan to add an on/off switch and a variable resistor to bring down the speed or turn it off if there is to much vibration.
Dew on the secondary mirror was a bigger problem then dew on the primary mirror, during the previous star party (2015).
And the white wiring? That is a nichrome heater (harvested from an old electric blanket) to heat up the air a wee bit. Maybe some airflow (fan) will do the trick.
* Plan B is to mount the fan on the edge of the scope tube and point it in the direction of the secondary mirror. What next?
Some links with more ideas
- How to install R-Sky dew heater on secondary mirror  Mabybe the best idea?
- Secondary dew heater / preventer for 12" Orion dob Some interesting facts to ponder about.