What I have found:
Answer goes from other field: astronomical observation. Phil was right: it should be some heating device. Here is a sample.
Super device from DewBuster and hand-made anti-dew system
Hand-made without heater (maybe it will work): Anti-Dewer: long tube, horn made from cardboard or plastic, it can be covered from the inside with black barhotom (only natural, it is better artificial), and outside paint matt black car paint for better protection from the elements. Get two in one: Anti-Dewer and blend together. Here is a sample.
We can use thin sheets of black foam :)