It's not so mysterious. We've all seen it: the droplets that form on the cool green grass of morning, or the sheen of moisture that forms on our telescope tubes as the chill of the evening air besets our observing session. But what can we do about dew? Who knew about dew? What's the big to-do about dew?

Moisture is in the air. The heat of the day causes more water to evaporate from the earth and the waterways. As night falls, this moisture, still in the air becomes more concentrated as the temperature drops. As it gets cooler, the humidity increases. When the humidity reaches 100% we get fog. But long before that, on surfaces that cool faster than the air (due to black-body radiation), this moisture condenses out of the air onto those cooler surfaces. This is dew!

Aside from the obvious affects, such as our clothing feeling damp, our paperwork getting moist with the possibility that our ink may start to run (this is called paper chromatography...), and water droplets actually forming on our telescope tubes and tables, what 's the big deal? Well the first obvious affect is that if it forms on your objective lens, primary mirror, or corrector plate you will get less light through the telescope and into the eyepiece. You will start to lose fainter stars and objects, and eventually, you will see NOTHING! I have had this happen to me. Another problem, more insidious and subtle, is that when droplets form on your optics, they collect dust from the air. When they evaporate later, this dust may be left as little circles. This causes the same problems over time, and may require alcohol and distilled water to remove (don't ever clean any of your optics dry...).

So what can we do about the dew? An ounce of prevention is worth a pound of cure. Dew Shields are our first line of defense. These increase the length of the optical tube on SCTs and refractors. This makes it harder for blackbody radiation to occur. (Deep space can't cool it if it can't see it.) But this only extends out time before the dew takes over. The best option is not let the optics cool down in the first place. Dew heaters are the answer. These are little thermal strips that you put on things that you don't want to become dew covered. They make them for eye-pieces, objectives, Telrads, and I have even seen one for lap-tops. I have even seen astronomers use them as body warmers on really cool nights. But no matter how much you plan and prepare, sometimes you lose the battle. To win the war on dew takes a serious tool: a hairdryer! (Or at least it looks like one...) They sell these specifically for astronomy. They will tackle the worst dew you can imagine. Remember though, you only want enough heat and wind to remove the dew. If you overheat your optics it will blur your images until they reach equilibrium again.

Clear skies and clear optics!