Dew Control Tips
It’s fall in Colorado, and that means more humidity in the air, and, if you aren’t mindful, dew on the telescope. This is particularly important for refractors and Cassegrain telescopes - SCTs and Maks, which have a large lens at the front that is highly susceptible to dew buildup.
Here are some tips to prevent and in the worst case, remove that dew.
Lens Hoods and Dew Shields
Dew shields are the first line of defense. These work well when you keep your objective pointing far enough towards the horizon that the lens isn’t exposed to the dew coming down from above. The issue of course is most of us want to point our telescopes much higher, making the passive dew shields less effective for dew, but still good light shields.
We carry a range of dew shields, but you can also make your own with some semi-rigid bendable plastic sheet, or in a pinch, some cardboard.
The best option is a combination of a dew shield with some active prevention, like a heater strip. AstroZap Flexi-Heat dew shields provide this best of both combination.
For a couple years, I’ve used a pair of chemical hand warmers with a little painters tape around the objective of my 80mm APO. This seems to work well at preventing dew for at least the four hours or so that the hand warmers last. Better would be wrapping an old sock around them to provide a bit of insulation. I use the same trick for my camera lenses when shooting wide field.
Dew Heater Strips
The professional solution is dew heater strips and lens hoods. Mile High Astronomy carries a range of these from AstroZap for 1.25” eyepieces, telrads, and finder scopes up to 11” and larger telescopes. We also carry dual channel dew controllers that allow you to vary the power draw and temperature of the strips. Shop our Dew Control Products.
I’ve got Dew, now what?
If you have a site with AC power, a hair dryer on the lowest setting, and held a foot or so away from the objective will clear away dew buildup in a minute or so.
Repointing the Telescope
If you don’t have a hairdryer but have dew buildup on your telescope objective, point the telescope down towards the ground. This will give the dew a chance to gradually dissipate on its own, but will take a while. Don’t take a cleaning cloth to the lens - this is likely to leave water mark streaks, and any dust that is on the lens sitting otherwise harmlessly could scratch the lens or the optical coatings. With telescope optics, particularly out in the field, the no touch approach is best.
If you have to pack up your telescope and there is dew on the lens, leave the cover off if possible to allow the dew to go away on it’s own. You don’t want to lock it in against the objective. If you need to place the lens cap on for safe transport, be sure to remove it again when you get home and let the telescope air-dry.
Beyond the Telescope
If you are an imager, you probably have a laptop with you. I find a medium sized moving box, with one end open and turned on it's side provides a good dew shield (and light shield) for the laptop and power strip.
For those with paper charts or log books, a clear plastic sheet to keep your items covered is a good idea. Be sure to similarly keep spare eyepieces and other items covered, otherwise they can easily get dewed over, too.