Because glass pointed at a clear sky radiates its heat into space and falls below the temperature of the air around it, at which point moisture already in that air condenses on it. It is not caused by the air being damp on its own.
Understanding the mechanism explains both remedies, and explains why the obvious one fails.
Radiative cooling
A surface facing a clear sky loses heat by radiation to space and becomes colder than the air around it. Once it falls below the dew point of that air, water condenses on it. Humidity matters, but the driver is the temperature difference the sky creates.
Why a shield works
A dew shield is a tube extension that narrows the patch of sky the glass can see, which slows the radiative loss and buys time — often a whole evening in mild conditions, and nothing at all on a bad night. It costs no power and belongs on every refractor and every folded tube.
Why a heater works differently
A heater band holds the glass a degree or two above the air, so the condition for condensation never arrives. It draws current all night, which is why a controller that varies the output is worth more than a larger band: running one at full power wastes charge and can put enough warmth into the tube to disturb the image.
Which optics dew first
Whatever faces the sky most directly. On a refractor or a folded tube that is the front element. On a Newtonian the secondary mirror often goes before the primary, because it faces the opening.
The questions that keep arriving
Does wiping it off work?
Briefly, and it risks the coating. The surface is still colder than the air, so it dews again within minutes. Prevention is the only approach that works through an evening.
Shield or heater?
Fit the shield first: on many nights it is enough on its own and costs no power. A heater band stops the process rather than delaying it, and is what a bad night needs.
Last reviewed 17 September 2026