DewThe 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.
Dew heaterA 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.