# Dew

Also called: dew point, dew heater, dew shield.

Moisture condensing on optics when they cool below the dew point — it fogs eyepieces, finders and lenses.

On clear nights everything radiates its heat to the sky and cools below the air temperature. When a surface reaches the dew point, water condenses on it. Refractor and SCT front lenses, finders and eyepieces are most at risk; a Newtonian's mirror at the bottom of its tube is better protected.

**At the eyepiece:** A dew shield (a tube extending past the lens) delays it; a gentle heater strip prevents it. Never wipe a dewed lens — let it dry or warm it.

**In AstroPilot:** Tonight's hour-by-hour forecast warns when dew is likely.

**Related:** [Transparency](https://astropilot.space/glossary/transparency.md), [Cooldown](https://astropilot.space/glossary/cooldown.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/dew