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.