Dos and Don’ts of Dealing with Dew

A dew shield-equipped telescope awaits as the sky darkens. Some simple devices and precautions can keep the dew at bay as the evening air cools. Credit: David Dickinson

Living and observing on the U.S. East Coast, I’m no stranger to dealing with dew on optics. In fact, doing the Universe Today Virtual Star Party back in the day, I would keep a hair dryer handy to periodically clear off the telescope. There are lots of passive dew shields and active heaters on the market today. You can also go low-tech and build your own shield.

Do-it-yourself dew shields are easy to make. Here a bungee cord keeps a rolled up sheet in place, keeping dew from settling on the telescope’s corrector plate. Credit: David Dickinson

How does dew form? On a clear, cool morning you can step outside to find your car dripping with water, even though it never rained the night before. Dew forms when the nighttime temperature dips below the air saturation temperature, known as the dew point. Condensation begins to collect on exposed surfaces, whether that’s a deck, car or telescope. Even in Arizona where it’s usually dry, I’ve had water dripping off the telescope tube on damp monsoon nights. Refractors and Schmidt-Cassegrain telescopes with exposed lenses and corrector plates are especially prone to this, although even Newtonian optics can fall prey to dew.

Unfortunately, some of the clearest summer nights are often also the dampest. What can you do about dew? First off, do not try wipe or scrape dew off once it has formed, as tiny particles of grit and dust can scratch precious optics.

The best course of action is to stop – or at least, slow down – the formation of dew in the first place. A passive system involves shielding optics while also insulating the glass and tube. This has the advantage of being easy to employ in the field, and does not require power. The disadvantage is that dew usually still wins out in the end after a long night of observing. To combat formation, you can buy ready-made dew shields (listed below). Or you can simply make one out of a sheet of one-inch thick foam (hiking mats work great), Velcro, and a bungee cord to keep it in place.

The insulating material of this sleeping bag pad not only shields the front optics from dew but keeps the tube from cooling. Credit: David Dickinson

Active systems place heating elements around optics, to keep them above the ambient dew point. These systems work similar to the defogger on your car’s rearview window, and can keep your telescope dew free for the entire evening. If you can get AC power out to your observing site, a hair dryer is fast and effective, with a few caveats. Be sure to use the hair-dryer’s lowest power setting, and aim it across the glass at an oblique angle to slowly evaporate dew. You don’t want to go at the objective lens or corrector plate with gusto, as it could potentially overheat and crack.         

Here’s what’s out on the market today for passive and active systems to combat dew:

  • Fairpoint dew shields ($22)
  • Celestron dew shields ($30) 
  • Astrozap Dew heaters (starting around $40) 
  • OPT dew controllers (starting around $16)
  • Svbony dew heater strip ($17) which I use on my 5-inch Maksutov telescope. It’s made for DSLR cameras, but also works great on small telescopes, and runs off of USB 5-volt power.

Don’t let dew force you to pack it in early, missing out on otherwise good conditions. With these techniques, you can successfully combat dew for the entire evening.

About David Dickinson

David is a freelance science writer, frequent contributor to Sky & Telescope and Universe Today, author of several astronomy books and long-time amateur astronomer. He lives with his wife Myscha in Norfolk, Virginia.

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