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.

Dew does not "fall" from the sky. It condenses from the surrounding air onto any object that's colder than the air's dew point. The dew point, often mentioned in weather broadcasts, depends on both temperature and humidity. When the humidity is 100 percent, the dew point is the same as the air temperature. At lower humidity, the dew point is below the air temperature. If it's below freezing, you get frost instead of liquid water.

An example of dew physics occurs when you take a bottle out of the refrigerator. If the bottle is colder than the air's dew point, it drips with condensation. Your telescope is the bottle.

"But my telescope can't get colder than the air!" a new Schmidt-Cassegrain owner once told me. "It was warmer than the air when I brought it outdoors. The Second Law of Thermodynamics says that can't happen!"

If only life were so simple. Objects do try to come to the same temperature as their environment and then stay there, as the Second Law says. But they don't exchange heat just with the air around them. They also exchange heat with objects at a distance by radiation. That's why the Sun can feel warm on your skin even though it's 93 million miles away. At night the heat flow goes in the opposite direction. The effective temperature of the dark night sky is just a few degrees above absolute zero, and a telescope in an open field is exposed to a whole celestial hemisphere of this cosmic chill.

The first line of defense against dew, therefore, is to shield your optics from as much exposure to the night sky as is feasible. The traditional dewcap extending beyond a refractor's lens often serves this purpose well enough to keep the lens dry. The longer the dewcap, the more likely it is to work. One of the nice things about a Newtonian reflector is that its entire tube acts as a dewcap to shield the mirror in the bottom. An open-tube reflector, however, needs a cloth shroud around its open framework to gain this benefit. The cloth itself, of course, will get wet on its sky-facing side.

The worst dew problems appear on exposed parts that are thin (or have low heat capacity) and rapidly radiate away their warmth. Schmidt-Cassegrain corrector plates are notorious for dewing; so are Telrad sights with their exposed glass. A dew shield is reportedly the first accessory that Schmidt-Cassegrain owners most often come back to buy.

You can easily make your own. A piece of tough 5/8-inch foam rubber, the kind sold in sporting-goods stores to go under sleeping bags, makes a dew shield that's cheap, durable, and very lightweight. The foam is an excellent insulator, for maximum effectiveness. If you're concerned that the cap might vignette the image (block some starlight near the edges of the field of view), you can cut the foam so it flares open at a very slight angle. A 3° opening angle should allow a 3° unvignetted field of view.

As a rule of thumb, a dewcap should be at least 1½ times as long as the aperture is wide. A side benefit is that the cap also cuts down on stray light getting into the telescope.

Eyepieces too are prone to dewing. Warm radiation from your face slows the process, but humidity from your eyeball and breath speeds it up. A tall rubber eyecup, the kind that extends above the eye lens all around, not only blocks stray light while you're observing but acts as a miniature dewcap when you're looking away.

The same principle works on large scales. Early on a clear morning, have you noticed grass in the middle of a field white with frost or dew while grass near a tree has none? The tree is a giant dewcap, and it can work for you too. If you'll be looking at only one part of the sky, it's nice to have trees around and behind you. Not just your telescope but your charts and accessories will stay dry longer.

Trees also reduce wind problems, but a slight breeze is a good thing. Radiational cooling is slow and inefficient compared to heat transfer with the surrounding air, so even the mildest breeze will keep your telescope nearly up to air temperature.

Then there's the observing umbrella, not a widely known accessory but one that works. A beach umbrella blocks the chill of absolute zero the same way it blocks the heat of the Sun. It can help shield all your gear and keep the chill off you too. On a still night a thermometer under an umbrella can read more than 10° Fahrenheit higher than when it is exposed to the open sky.

The Heat Is On

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 and other anti-dew systems that run off a 12-volt battery are advertised regularly in Sky & Telescope magazine.

Warmed optics can have unexpected benefits. Dew works its first subtle evils before you notice anything. The late Walter Scott Houston used electric warmers on both the objective and 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 dewy.

"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. After all, the whole idea is to stop it from growing colder than the air.

Not-So-Cold Storage

The most destructive dewing happens when a telescope is in storage. No telescope should be closed up and put away until it is thoroughly dry. Water with nowhere to escape, or condensation that forms and evaporates repeatedly in a sealed environment over months and years, may attack optical coatings and ultimately etch the glass itself.

How, you may ask, does water get into an airtight space that was dry when you sealed it? The answer is it was there all along. Air contains water vapor, and if your telescope gets colder than what the dew point was when the air was sealed in, water will condense. This is why so many puzzled telescope owners discover water stains on the inside surfaces of their corrector plates and refractor lenses.

Several approaches can prevent this. Don't move a sealed telescope from warm to cold storage. In fact, sealing may be a bad idea altogether. The best telescope covering is cloth, which will "breathe." It keeps dust off but lets water vapor out. And you might want to leave the eyepiece holder covered only with cloth, just enough to keep dust and spiders out.

The worst problems occur when a warm front of humid air blows in after cold weather, as often happens in early spring. Everything cold gets drenched. A cloth wrap may be the best defense here too; it will greatly reduce the amount of humid air that can flow over cold parts.

The usual advice is to store a telescope at the outdoor temperature to minimize tube currents when you set it up. But this old rule may need modification. Keeping the telescope a little warmer will tend to thwart condensation. An enclosed porch or attached garage may provide the extra few degrees you need. And really long-term storage should probably be inside your living space. Never leave a telescope in a damp basement or garage or, as a rule of thumb, any place where tools grow rusty.

You can take active countermeasures too. A 4- or 7-watt light bulb inserted into a blanketed telescope makes a nice low-power heater. Position it just below or right next to the glass, or else you may merely drive off water from other parts of the tube that will condense onto the cold optics. Running the bulb continuously will cost about a dollar per watt per year. You might turn it on only in the damp season, or attach it to a humidistat switch.

Silica gel desiccant will dehumidify the air in a tightly sealed enclosure. I keep a ¾-pound bag in plastic webbing attached to the inside of one of the tube caps of my 12.5-inch reflector. Every month or two when the bag's indicator slip turns from blue to pink, I heat the bag in a toaster oven in my observatory to drive off the collected moisture. The more tightly you seal your tube or storage case, the less often you'll have to do this. Silica gel is available from many sources.

Water can be an insidious enemy for astronomers, but a little knowledge will keep it permanently at bay.

Alan MacRobert is an associate editor of Sky & Telescope magazine and an avid backyard astronomer.