Dewy or Don’t We?

I have been so drawn to this title that has been floating around in my head that I just had to start writing about it. You may find the idea of anything floating around in my head sufficiently amusing that there is no need for you to read any further.

Dew season is coming up, so we may as well get ready for it. I have done some reading on dew, but I am not yet at the expert level. My aim in writing on this topic is help me clarify my thoughts, and to help raise awareness and stimulate thought in a manner that will help astronomers meet, understand, and defeat dew.

The dew point is one of the keys to understanding dew. The atmosphere always contains moisture, sometimes more of it than others. As the temperature drops from daytime into nighttime, the atmosphere is able to hold less moisture than when it is warmer. The dew point is a function of the amount of moisture in the atmosphere, and temperature. This function yields a temperature at which the atmosphere is “forced” to begin releasing moisture as water droplets.

But this is not the final answer. Astronomers must deal with the “chilling effect” of the night sky (that we are out to observe) on their equipment. Dew does not “fall” onto the upper surfaces of our equipment, it forms there as a result of the chilling effect. Our equipment radiates heat (IR radiation) in all directions. The bottoms and sides of our equipment gets some heat back in the form of radiation from our surroundings. The clear air above us, however, does not give back nearly as much heat as our surroundings on the the ground. This causes the upper surfaces to chill faster, which in turn hastens the formation of moisture on these surfaces.

How can the weather forecast help us? I don’t know exactly, but here is a possibility. I have observed for sometime that the forecast usually predicts that during the daytime the air temperature will be well above the dew point. On most nights, and for most of the night, the temperature and dew point are about the same. Perhaps when the air temperature falls to the dew point (and probably a few degrees above that point) is when dew protection is needed. In the future, I intend to note the time in the forecast at which the air temperature drops to the dew point, and observe when dew actually starts forming. If I am in the back yard, next to my weather station (Google: Weatherunderground KMDASHTO6), it is easy to monitor conditions.

Here is another exploitable fact: both glass and painted surfaces radiate heat very well, and are both prone to collecting condensation. Using painted metal surfaces as a proxy for glass, even if we have protection in place on the glass, we can gain a sense of when and under what conditions dew forms.

A dew shield is the first line of defense against dew forming on our optics. When a telescope is not pointed straight up, dew shields give us some protection from dew. Not because they prevent the dew from falling onto the glass, but because they are a barrier between the forward facing optics and the chilling effect of the night sky above. Shields do not help much when observing near the zenith.

A slight breeze can be helpful. This is not because of a “drying” effect of the breeze, but because air that is kept slightly warmer by radiation received from our surroundings is moving over the optics. A dew shield, counterintuitively, might prevent the breeze from “warming” our optics and could actually increase the likelihood that dew will form.

The next line of defense is applied heat. It doesn’t take much heat. The optics that we are trying to protect need be only slightly warmer than the surrounding air, and not necessarily warm to the touch. Three ways to warm optics come to mind. Dew heaters, usually as strap connected to a controller that controls the amount of heat applied (and the drain on valuable and heavy battery capacity) are convenient. Probably best monitor the ambient temperature relative to the dew point and begin applying heat before the dew ever forms to avoid it all together. Otherwise one might end up waiting some period of time for optics to clear after the dew has formed. 12v hair dryer type devices are used by some astronomers, but periodic reapplication of heat is required. I have heard of astronomers using rubber bands to affix chemical hand warmers to Telrads. 

Here is an experiential data point: I turned on an Astrozap dew heater strap and controller on my 4″ refractor AFTER dew had begun to form on the objective lens at the during a photography session on October 14th. It took about 15 minutes with the power setting at full/high for the dew to clear. I turned it down to about 30% and the lens stayed clear for the remainder of the session. In the future, I plan to turn on the dew heater earlier (guided by forecast and/or actual conditions), and experiment with even lower settings. Perhaps there is some factor that I can consider that will suggest whether a lower or higher setting is appropriate.

There are many finer points to shielding and heating solutions, but I think that this a good start with regard to understanding and solving the dew problem. I think that the key to being more effective in combating dew is to be observant of forecasts, weather conditions in real time when possible vs. when dew actually forms on our equipment. Also, gaining experience with how much heat to apply (or how much of an amperage load to allow on our batteries) to keep dew from forming will be helpful as well.

© James R. Johnson, 2021.

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