Sometimes when using my telescope late at night or early in the morning, especially in the more humid of seasons, I notice i sometimes get dew buildup on the lenses.

How can I minimize damp buildup whilst using my telescope?

Simply dabbing away the liquid with a dry cloth doesn't seem to do the trick as I am still left with some water tracks and residue that I just can't soak up, this then means that all of the light is refracted providing a very fuzzy and blurry image.

What preventative measures can i take to stop damp build up in the first place?


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.

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  • $\begingroup$ +1 for "Your telescope is trying to get into thermal equilibrium with outer space." $\endgroup$ – agtoever Aug 28 '14 at 21:04

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