Dealing with Dew

By Pierre Martin

Sooner or later, it happens to all of us. It is a perfectly clear night, the transparency is excellent, the wind is calm and you look forward to enjoy a spectacular night under the stars. Within only one hour of using your telescope, you realize something is not quite right. Images through the eyepiece now seem dimmer and fuzzier, yet the sky is still excellent. Is your telescope not working right? You take a closer look at it, and realize it is all covered with dew, including the main lens and eyepieces. Disappointed, you realize there is nothing more you can do. You must cut your session unexpectedly short and head back home.

That Pesky Dew

Dew is one of astronomer's worse enemy. To owners of Schmidt-Cassegrain and refractor telescopes, this is particular bad since the main lens or correcter plate is more exposed to open air. Dew can be expected on the clearest nights when the air temperature changes. During winter, this is known as frost. Some nights can be worse than others, all depending on the level of humidity and air temperature.

Can anything be done to counter the frustration of a dewy night? Absolutely! The first step in the battle is to zap the dew. A dew zapper (or hair dryer) plugs into a car cigarette lighter. It blows warm air on the optical surface and dew will temporarily disappear for a while. A better solution is to get a dew cap. A dew cap is basically a tube extension that fits on the front end of your scope. Ideally, it should be at least twice the diameter of your scope to work efficiently. The main purpose of this simple device is to create a "pocket" of warmer air in front of the lens and have it trapped there long enough to slow down any dew that might creep inside. A dew cap also serves to help keep unwanted stray light from entering the optical system. Dew caps come in many varieties from manufacturers. Some are even heated to be more effective. Some manufacturers sell all metal dew caps, but these can be rather expensive. Their extra weight can also generate problems on a mount. I find a better solution is to simply build one yourself! It is surprisingly easy to do, and might even work better than those fancy commercial ones. After seeing an article in Sky & Telescope magazine, I decided to build a dew cap with some very simple material for a few bucks, and up to now, it has worked wonderfully.

For my Celestron C8, I chose a lightweight strong blue foam from the local camping store (the same kind widely used as a mat for sleeping bags). I cut the foam to match the exact outer diameter of my telescope. It must fit snugly on the front end of the scope to stay in position. To avoid any vignetting (obstruction in the scope's light path), I cut the foam to get a 5 degrees opening. The material was glued together using a hot glue gun. I added some extra layers of foam and glue on the front end to reinforce the strength and insure the cap would preserve its round shape. Finally, I painted the dew cap in flat black to match the color of my scope, and to reduce reflections of stray light from going inside the optical system. The result is a dew cap that is lightweight, cheap and durable. The foam acts as an excellent insulator and in my opinion, it works better than any other material I have tried before.

I also made good use of the foam material to create a simple "box shaped" dew shield for my Telrad viewfinder (with great results!). The Telrad is a zero power finder with a bullseye pattern projected on a glass. This permits easy and quick aiming of the telescope across the sky. Without some sort of shield or insulation, the Telrad's exposed glass can often get covered with dew and become unusable.

  

Lastly, I built a foam container that I stick (double sided tape) on top of my mount. Inside the container, I store all my eyepieces, filters and accessories that I plan to use for the night. That way, everything stays dry all the time and is always within easy reach.

That's about all the arsenal I have to combat dew. With such simple low-cost gear, I have been satisfied with the results on most nights.Sometimes, dew caps DO have their limits.

More arsenals

On VERY dewy nights and when you want to use your scope for extended periods of time, they might fail to do the job after all. When that happens, it may be worth to consider getting a set of low voltage heaters that wrap around the front end of the scope (where the optical lens is located). These heaters produce a very mild heat that will be just enough to keep dew from settling on the optics. These heaters are available in different sizes to fit various accessories such as eyepieces, finderscopes, guidescopes among others. Some systems will even let you adjust the level of heat according to how bad the conditions are. Such a system is sure to keep your optics completely dry even if your telescope is dripping wet, no matter how long you want to observe!

Depending on your observing program, it's up to you to decide how much protection your telescope needs. With your telescope ready and well protected, you no longer have to endure losing the battle against nature's pesky dew!