Passive anti-dew system (i.e., requiring no power like a lens/corrector hood) may be able to help but only up to a certain extent (it delays the formation of dew, perhaps for a few hours, but will not completely stop it). As soon as the temperature of the scope drops below that of the dew point, dew will start to form and you will have no choice but to end your observation early. Without an active anti-dew system (heating by using electricity), it is simply impossible to completely eliminate dew. In this article, I will describe how an inexpensive DIY dew heater could be constructed using nichrome wire as the heating element.

DIY dew heater using nichrome wire

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