I have a homemade dew zapper on my Unitron 4" refractor. The Main objective, eyepiece, Finder objective and eyepiece are all protected. Before installing the heaters my observing session on any resonably moist night would end after about half and hour. After installation no dew all night, even on the most moisture laden atmosphere.
You just have to be careful not to turn the heater control up too much.
Where you set it will come with experience. Cheers, John
