A heater replaces the lost heat and prevents cooling and thus fogging. The difference of the temperatures is very low, so there are no problems with tube seeing.
PegasusAstro DewMaster - Digital Dew Heater Controller, 5 Channel
Connect up to five dew heaters and control them individually using PWM duty cycle. Current and voltage are measured, an optionally available external sensor enables automatic heating control.
5 independently controlled channels with up to 5 A each
Pulse Width Modulation duty cycle control for reduced size and consumption
Intuitive operation: Select output by pushing the rotary knob, adjust it by rotating the rotary knob
2 pass-through outputs for supplying additional equipment
Input voltage and output amps are measured
Red LED display for general information like measurements
Red status LED for quick check
External evironmental sensor is optional, not only allowing measurements, but also automatic heater control!