The moisture that settles on optics when they cool below the dew point by radiating heat to the sky. It strikes exposed surfaces first: Schmidt corrector, refractor lens, finder, eyepieces. Do not wipe it: a dew shield or a dew heater prevents it, a hair dryer clears it.
Related terms
- Dew shield
A tube extending the front of a refractor or Schmidt-Cassegrain, about 1.5 to 2 times as long as the aperture. It limits how much the optics cool towards the sky and delays dew. It also blocks stray light from street lamps.
- Dew heater
A heating strap wrapped around an optic and run from 12 V, keeping it a few degrees above the dew point. Essential for a whole night in damp weather, especially when imaging. Set it to the lowest useful level so it does not create air currents.
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