Dew is the commonest way a clear night ends early. It forms on the optics before it forms on anything else, and the reason is a piece of physics that also explains what to do about it.
Why optics dew first
An object exposed to a clear sky radiates heat away to space and cools below the surrounding air. Optical surfaces face straight up, are thin and are exposed, so they cool fastest and fall below the dew point first. This is why a corrector plate fogs while the ground is still dry, and why it happens on the clearest nights.
The dew shield, which is free physics
A tube extending in front of the optic restricts its view of the open sky, so it radiates less heat away and stays warmer. A dew shield alone buys an hour or two on most nights and can be made from a rolled sheet of foam. Newtonian reflectors have a built-in one, which is why their primary mirrors rarely dew and their secondaries sometimes do.
The heater, which is the reliable answer
A dew heater strap wraps around the tube near the optic and holds it a degree or two above ambient, which is enough to prevent condensation entirely. It needs a power supply, usually a battery pack, and a controller so it uses the least power that works. For a catadioptric in a humid climate it is not optional.
What not to do
Do not wipe a dewed optic: you will smear it and risk scratching the coating. Do not point a hairdryer at cold glass, which risks thermal shock on a corrector. If dew forms and you have no heater, cap the instrument, bring it indoors and let it dry in still air.
Questions people ask about dew heater
Why does my telescope fog up?
Radiative cooling: the optics face the open sky, cool below the air temperature and fall past the dew point, which is why they fog on clear still nights.
Do I need a dew heater?
In a humid climate with a refractor or catadioptric, yes. A newtonian with a long tube often manages with a dew shield alone.
Can I wipe dew off a telescope?
No. Wiping smears the film and risks the coating. Cap the instrument and let it dry indoors in still air.