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.
A Dew Primer
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 the dew point is below freezing, you get frost instead of liquid water.
A familiar 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 distant objects by radiation. That's why the Sun can feel warm on your skin even though it's 93 million miles from your skin. 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.
Shield Your Optics
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, shielding the primary mirror at the tube's 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 happen 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 the (otherwise excellent) Telrad sights with their exposed glass plates. 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 (16-mm) 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. As a rule of thumb, a dewcap should be at least 1 1/2 times as long as the telescope's aperture is wide. (An example is illustrated on the first page of this article.) A side benefit is that the cap also cuts down on stray light getting into the telescope. That being said, 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 slight angle. A mere 3° opening angle should allow at least a 3° unvignetted field of view.
Eyepieces are prone to dewing too. Warmth radiating from your face slows the dewing 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.
Shield Your Site
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 just one part of the sky, try to have trees around and behind you. Not only will your telescope stay dry longer; so will your charts and accessories.
Trees also reduce wind problems, but a slight breeze is a good thing: it will keep your telescope nearly up to the ambient air temperature, since radiational cooling is slow and inefficient compared to heat transfer with moving air.
Then there's the observing umbrella, not a widely known accessory but one that works. A beach umbrella blocks the chill of outer space the same way it blocks the heat of the Sun. It can help shield all your gear and you too from the cosmic deep freeze. On a still night a thermometer under an umbrella can read more than 10° Fahrenheit (6° Celcius) higher than when it is exposed to the open sky.
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.
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 that has nowhere to escape may attack optical coatings and ultimately etch the glass itself. So may condensation that forms and evaporates repeatedly in a sealed environment over long periods of time.
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 can "breathe." It keeps dust off while letting 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. 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 objective, or else it may merely drive off water from other parts of the tube and send it condense onto your 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 my 12.5-inch reflector's tube caps. 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. I got mine from Hydrosorbent Products (www.dehumidify.com; P.O. Box 437, 25 School St., Ashley Falls, MA 01222 USA; 800-448-7903 / 413-229-2967).
Water can be an insidious enemy for astronomers, but a little knowledge will keep it permanently at bay.