You can easily make your own foam-rubber dew cap. Sky & Telescope's Dennis diCicco created this one by cutting a sheet of foam with a knife and ruler, butting its cut edges together, and joining them with hot-melt glue. After decades of experimenting with other materials, he calls it the best dewcap he has ever used.

S&T / Dennis di Cicco

The most common equipment hassle that observers face at night is water on the telescope. This comes as a surprise to people who expect things to stay dry in clear weather. Unfortunately, the steadiest, sharpest telescopic views are often had under precisely the atmospheric conditions that cause dew to form. At the eyepiece you first notice dim stars and galaxies becoming harder to see, then bright stars develop fuzzy halos — and a check with the flashlight reveals wet haze coating the optics. In severe cases the whole telescope may be soaked. Wiping never helps; more water condenses the moment you stop. At this point many observers pack up, defeated.

However, you can keep your lenses and mirrors crystal clear in even the heaviest dewing conditions. You just need to understand the enemy and take effective countermeasures.