…continuedDealing With Dew
Warming Your Optics
There will be times and places where none of this is enough. You then have no choice but to warm your optics, usually electrically.
A 120-volt hair dryer (used gently from a distance so it doesn't overheat the glass and warp it) will blow off dew for perhaps five minutes. Then you have to use it again. And again. A 12-volt auto windshield defogger gun is somewhat less effective.
A better way is to apply a little heat continuously. Heated dewcaps that run off batteries have been advertised and reviewed in Sky & Telescope. (An elaborate example is the Kendrick Dew Remover System; May 1994 issue, page 52. A somewhat similar system is sold by Orion Telescopes & Binoculars.) If you're comfortable working with electrical parts and a soldering iron, you can make an antidew heater to any size, shape, and specification you want.
Warmed optics can have unexpected benefits. Dew works its first subtle evils before you notice anything. The late S&T columnist Walter Scott Houston used electric warmers on both the objective and the eyepiece holder of his 4-inch refractor. When he turned off the power, the telescope could lose a whole magnitude of light grasp before the objective actually looked damp. "Even on nights when dewing is not noticeable," Houston wrote, "the star images seem better with the heaters on than without them!" This may be because, contrary to what you might think, gentle heating keeps a telescope close to the temperature of the surrounding air, minimizing poor "seeing" caused by air-temperature differences near your optics. After all, the whole idea is to stop the telescope from growing colder than the air.