Dealing with damp and dew whilst stargazing at night or in the early morning
Condensation will build up because the temperature of your optics has fallen below the dew point.

You simply have to keep the temperature your telescope above the dew point, which can be quite difficult on humid nights. Dew shields can help, as can dew heaters (which literally just gently heat your telescope and/or eyepiece).


  Hypothetical: "I'm looking at the graph of dew point vs air temperature on the link above. I don't understand how my optics can get colder than the air temperature!


At night, our source of heat (the Sun) has disappeared. The unlit side of the Earth (and everything on it) begins radiating its latent heat away into space (which has a temperature of ~3 Kelvin). Your telescope is trying to get into thermal equilibrium with outer space.