Dew heaters or heater pads are telescopes accessories used to prevent dew from forming on the telescope’s lens. During long imaging sessions, it is not uncommon for the main lens of refractors to form dew. A heater is used to keep the objective lens at a temperature a few degrees C above the dew point to prevent the formation of dew.
I used nichrome wires from a local electronics store to build several DIY heater pads for my telescope, which I find useful in keeping the lenses free from dew especially when imaging in remote observing sites.
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Night Sky in Focus © Anthony Urbano | Bacoor, Philippines