dew forms on telescope optics when their temperature falls below the surrounding atmosphere's “dew point” which is dependent on the atmosphere's humidity level
this creates a problem for astronomers who need to allow their telescopes to cool to surrounding temperatures to minimise internal currents which compromise "seeing" and also need to cool any CCD or digital camera to minimise CCD noise in long exposures.
once it has formed on your optics, you either have to stop your session for the night or gently use a 12V hair dryer.
Why does dew form?
“Dew Point Temperature” is the temperature at which, for a given mass percentage of water in ambient air, the air becomes fully saturated (100% Relative Humidity) and liquid water may condense.
Through infra-red radiative cooling, your telescope can actually cool faster than the ambient air. In humid conditions when the air is nearly saturated with moisture anyway, your telescope can reach the “Dew Point Temperature” before the air does.
Preventing dew from forming:
know the dew point and use telltale indicators for how aggressive you must be - prevention is better than cure.
Grass, car windows, and any exposed metal parts will signal the dew and frost monster early.
Dew does not form as quickly when air is moving (breeze).
choose a site where the dew point is unlikely to be reached eg. low humidity (this is also good for sky transparency and seeing)
choose a night with a little breeze as this tends to minimise dew
slow the radiative cooling of the optics but avoid telescope being too warm which will adversely effect seeing:
Cover the objective or corrector plate when not in use; pointing the telescope down will also help. Avoid pointing the telescope upwards.
dew shields - esp. needed for telescopes with lens at its end such as SCT's & refractors
Reflectix type roof insulation (foiled bubble wrap) has been very good, and it does not hamper cooldown enough to hurt wavefront quality. Line the shade with black felt to stop reflection from the foil.
use a “dew heater” to make up the heat lost to radiative cooling and thus, keep the scope a degree or two above the air
temperature.
Kendrick is one big name.
You could also check a very good controller (The Dewbuster) which sells for about $149.95. It is a pulse controller with sensors that measure the temp of your OTA and the ambient air and then pulses the current to keep the OTA just above the dew point temp. There are also diagrams and instructions for making your own heating strips to use with the controller.
If you can't afford a commercial dew heater, effective home-made dew heaters can easily be made with a bag of cheap 1/2 watt resistors and a soldering gun.
Keep all accessory optics in a closed poly case when not in use. Add a chemical pocket warmer if needed. Cover your eyepiece or binoviewer with a towel when not in use. Those nifty eyepiece holders should only be used in the desert! :)