Much Ado About Dew

Jack Kramer

Cool crisp fall nights are often great for observing, but dew can be especially troublesome at this time of year. Besides the obvious fogging of your optics, how does dew do what it does?

Dew forms on exposed surfaces that are cooler than the dew point of the surrounding air. Telescopes lose a lot of heat and often end up at a slightly lower temperature than the ambient air. Since the cooling is due to radiation of heat to the night sky, optical surfaces open to the sky are most likely to dew up first. This can be delayed by reducing their exposure to the sky. Dew is not as much a problem with solid tube Newtonians, because the primary mirror is at the bottom of a long tube. But you can still get dewing on the secondary mirror and particularly on the finder. Dew is more a problem with refractors, Maksutov, and Schmidt-Cassegrain designs because the glass is much more exposed to the air. Refractors come standard with dew shields (or "dew caps"), but invariably dew caps are extra-cost accessories for SCTs and Maksutovs. That makes no sense at all, because SCTs and Maks are notorious for dewing up. It is somewhat ironic that the point at which your optics start dewing up is also the point at which they're working at their best, since they have fully acclimated to the outside temperature!

Dew shields (and the shroud around a truss tube Newtonian mirror) reduce the optics' exposure to the air, but they are a delaying tactic, not a cure. However, a well-designed dew shield can delay the settling of dew until you're finished observing for the night. One rule of thumb is that the length of the dew shield should be twice the diameter of the primary lens or mirror. Also, if the diameter of the dew shield is sufficiently large, small silica gel packets can be mounted inside the shield with Velcro to help lower the amount of moisture in the air immediately surrounding the lens or mirror.

Matt Tarlach in California tells about the results he has obtained from improving his dew shields:
- "Lining an aluminum dewcap with felt or flocked paper increases its insulating properties substantially. It also helps suppress stray light; the improvement is especially noticeable when observing in bright, street-lit environments.
- Covering the outside of the dewcap with aluminum foil or aluminum duct tape (real aluminum tape, not the more common gray "duct tape") slows radiation of heat to the sky.
- Many dewcaps, especially on finders, are too short to be effective. Even some premium refractors have dewcaps that might be described as vestigial. I have made a couple of dewcap extensions from 20x28cm sheets of thin (2mm?) closed cell foam, available in craft stores under the brand name of "Foamies." This material is easily cut and rolled into a cylindrical dewcap shape, has enough stretch and grippiness to make a secure slip-fit over the existing dewcap, has good insulating properties, and is quite durable."

An alternative is "dew zappers" (heaters), which apply a tiny bit of heat near the optical components to keep them just above the dew point. In this respect, heaters are better than simple dew shields because they actually prevent the problem. In the winter this can be especially important because condensation can freeze into a thin coat of ice. The downside is that heaters consume a lot of electric power, normally supplied from some sort of portable battery pack.

What about the effect that the color of a telescope tube has on the time it takes to acclimate the optics and on fogging from dew? When marketers design a scope, they probably choose a color they feel is appealing to their target consumer rather than considering the color's functionality. If you have a choice, what should it be? It's common knowledge that a dark color absorbs and holds heat. But a light color reflects heat and light, so if it's setting outside in the sunlight during the day, it won't get as hot as a scope with a black tube. If the scope is warmer than the cool nighttime air, then the black surface is a more efficient radiator than a white one and helps the optics adjust more quickly. But if it radiates too well, eventually it cools faster than the outside air, then transfers this coolness to the optics, which fog up. One comparative experiment on dew shields revealed that a silvery exterior minimized dew formation.

Canadian observer Geoff Gaherty reflects on how color affected dewing of his optics: "I just did a survey of the sixteen scopes I've owned, past and present. Five had black tubes, five had blue, three had white, three were 'other'. I can't say I've ever noticed any difference." So we're driven to conclude that any color probably is okay. The bottom line may not be so much a matter of how well the color of the telescope tube reacts to temperature and humidity conditions, but how easy it is to see in the dark. In that case, white is the winner.

Published in the November 2004 issue of the NightTimes