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.