Best Dew Heaters and Controllers

Dew heater straps are sold by the diameter or circumference of the optical element they wrap. The key measurement is the outside diameter of the dew shield or lens cell, not the aperture. Most brands offer sizes for 1.25-inch and 2-inch eyepieces, SCT corrector plates (8, 10, 12 inch), and free-form straps you cut to length for unusual sizes. Measure the outside circumference of the tube or cell before ordering.

Running at full power wastes battery and can over-heat optics enough to induce thermal currents in the image. A controller uses pulse-width modulation to run heaters at exactly the power needed to stay above the dew point. An intelligent controller with a humidity sensor adjusts automatically throughout the night, which matters when temperatures drop several degrees between midnight and dawn.

Count your dew-sensitive surfaces: main objective or corrector plate, finder scope, guide scope, eyepiece or camera train, and any external flat panel. A dedicated astrophotography rig with a refractor, guide scope, and flat panel needs three to four outputs minimum. A Dobsonian used visually might only need one. Buy one more channel than you think you need, because the list always grows.

A single mid-sized dew heater running at 50 percent draws roughly 3 to 7 watts, which is minor compared to a mount and camera. A full four-heater setup plus mount plus camera draws 40 to 80 watts depending on equipment, so a 200 Wh portable power station lasts only two to three hours. For all-night sessions you need at least a 500 Wh battery, or mains power.