…continuedDealing With Dew
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.