To Dew or Not to Dew: Thatís the question, dewde!

By Steve Walters, 1999

Earlier this year, I realized that combating dew has been a continuous saga in my astronomical pursuits for over 20 years and that I really did not understand dew in even a pseudo-scientific way. This prompted me to do some technical reading to learn more about dew and how to control it. I donít claim to be an authority, but this little article documents what I have learned about dew and its prevention. I hope it can help you enjoy dewless observing.

What Is Dew?

There are a lot of myths about dew floating around out there. Some people think that dew falls from the sky like rain. Others say it rises from the ground. Some astronomers swear by their dew caps while others swear at them. And, of course, the Kendrick Dew System is the legendary preventive but there are those who carry hair dryers around with their telescopes. Here are the simple facts about dew.

Air contains moisture. The amount of moisture in the air varies according to the airís temperature where warm air can hold more moisture than cold air. The moisture in the air and the air itself are at the same temperature. Letís call this Tair. The amount of moisture in the air is indicated by the Relative Humidity (RH) which can vary from 0% in completely dry air to 100% in completely saturated air. RH is a measure of how close air is to saturation.

A common myth is that dew cannot form when the air is warm. This is not so. Dew can form at any air temperature and at any relative humidity level other than zero. But, as we will see, it becomes easier to prevent dew when the humidity is low. So itís a good idea to learn about your observing climate and what itís like where youíre going if you plan a trip to a remote location.

Now, letís introduce an object such as a telescope into this moist air. The scope is at some temperature, letís say Tscope, which for various reasons is not the same as the air. There is a temperature which is called the Dew Point (Tdew) at which invisible moisture in the air will condense into water droplets on any object that is at or below this temperature. If Tscope is at or below Tdew, water will materialize on the scope. So, there are three variables: Tair and RH (which allow us to determine Tdew) and Tscope. The Dew Point can be calculated using the following formula:

Tdew = (112 + 0.9 Tair)*B Ė 112 + 0.1 Tair where B = 8th root of (RH/100) and all temperatures are in Centigrade.

This may seem complex, but attached is a table showing the dew point for various air temperatures and relative humidity levels. Take a look at the table. For example, at a RH of 50% and an air temperature of 40 degrees, the dew point is 23 degrees but if relative humidity rose to 90%, the dew point would be 37 degrees. If you examine the table, you can see that as RH approaches 100%, Tdew approaches Tair. This means at high humidity, the dew point is close to the air temperature. At low humidity, Tdew can be much lower than Tair. But at 100% RH, the dew point is the same as the air temperature and fog will occur.

An interesting fact that can be observed is that as night falls, the air temperature drops and the relative humidity rises. If we examine our table, we see that these tend to offset each other regarding the dew point. For example, at 50 degrees and 50% RH, the dew point is 32 degrees. If the temperature drops to 45 degrees while RH rises to 60%, the dew point remains constant at 32 degrees. In actuality, dew point does not usually remain constant as night falls, it can rise or fall depending on which parameter changes fastest but, in general, dew point will not change as fast as the air temperature due to this effect. For telescope users, this means that when the temperature drops, you have to watch your scope closely as it could quickly drop below the dew point.

Another common belief is that if you keep your scope just above the air temperature, dew will not form. This is true, however you may be keeping your optics much warmer than necessary which will lead to distortion, greater air currents in the tube and wasted power leading to shorter battery life (and shorter observing sessions, boo hoo). The simple fact is that moisture will condense on any object at or below Tdew so our goal is to never allow our optics to get below Tdew.

Dew Prevention

Now, letís understand what is going on with our telescope in this moist air. The telescope is radiating heat into the night sky. The night sky is a terrific heat sink since itís at 3 degrees Kelvin which is nearly absolute zero. Your telescope is sending heat from its optics into space. The area of space "seen" by the optics depends on the telescope configuration. For example, an SCT corrector plate "sees" nearly the entire sky, a rather large area, while a dobsonian mirror "sees" only the area of its open tube. As heat leaves the optics, the opticsí temperature will drop. When the temperature of the optics reaches Tdew, your observing is over. Moisture will immediately form on the optical surface and you wonít be able to see. This is why SCT owners frequently top their scopes off with dew caps. A dew cap limits the angle at which heat can be radiated so the cooling process will slow and it will take longer for dew to form. Sometimes, this can be enough for an observing session but eventually, moisture will condense on your corrector plate if the dew cap is your only cure. It will simply take longer. Another trick to delay dew formation is to point your scope at the ground or cover it when youíre not using it. If dew forms on your optics, this can also help speed the recovery.

Next, enter the Kendrick Dew System, the Orion Dew Zapper and all those little homemade resistor ring beasties. These little gizmos all address the problem directly: they attempt to keep the optics above Tdew. Lots of folks get these things, strap them on and turn up the heat. Sure enough, they donít get any dew. They have solved one problem while creating two others. If the optics are over heated, they can produce distorted images. Dobsonians and newtonians will have excessive tube currents as the heat from the primary moves up the tube drawing cold air in behind. Pouring more heat into your optics than necessary also means shorter battery life. Incidentally, I really advise you NOT to power dew heaters from your car battery especially in remote locations like the Australian Outback unless you enjoy walking.

So, we would like to find a way to know how much power to put into the heaters to keep the optics just above Tdew but not to over "dew" it. Fortunately, there is a cheap solution at your local Radio Shack store (for the record, I have no financial interests in Radio Shack). They sell a digital indoor/outdoor thermometer ($25) that also indicates relative humidity. By simply getting the "outdoor" sensor in good thermal contact with the optics (I made a metal block that encloses the sensor and is bolted to my corrector plate housing), you can know the temperature of the optics while the "indoor" sensors show the air temperature and RH. By looking up Tdew in the table from the Tair and RH indicators, you can know whether to give the scope more or less heat. Often, you can delay for a considerable time before turning on your heaters. Professional observatories have systems which measure these parameters and decide when to heat and how much. You can do it too for only $25 but you have to be part of the system! Just donít cut it too close as the accuracy of these sensors is less than perfect.


Ok, hereís the cheat sheet for your final exam:

  1. Dew forms on any object with a temperature at or below the Dew Point.
  2. Dew Point depends on air temperature and relative humidity and can be predicted
  3. Dew Point gets closer to the air temperature as relative humidity rises
  4. Dew caps simply delay the inevitable but sometimes this can be enough
  5. Dew heaters can completely prevent dew
  6. Dew heaters can be controlled to minimize distortion and maximize battery life

Never again should we find dew on a S*T*A*R memberís scope! Be a dewless "dewde" from S*T*A*R!

Clear Skies!


Update (3/2000) - Since writing this article, I have built a controller that senses the air temperature and the optical system temperature and compares them. Using two thermisters, an op-amp, a transistor driver, a relay and a simple potentiometer, I can control the relative temperature offset between the air and the telescope. The controller simply turns the heaters on whenever the optic's temperature is lower than the air temperature plus the offset. This little beauty has kept my optics dew-free for 3 years now. You will need to use the Radio Shack dual thermometer to see what is going on. Otherwise you'll have no idea how hot you're making your optics.

Update (6/2004) - I modified the Dew-Not system to use a power FET instead of a relay. This saves both space and power in the design.

Here's a schematic.

Kudos to Frank Loso for locating the formula for dew point and for reviewing this article. Thanks Frank!