Dew is a familiar problem for amateur astronomers. Anyone who has left a telescope outside on a humid night has probably encountered an objective lens or corrector plate covered with moisture before morning. The standard solution is equally familiar: install a dew shield and a dew heater.
Those recommendations make good sense for a telescope sitting outside all night in your backyard. But a telescope in a remote observatory with an automated, roll-off roof presents a somewhat different situation. The observatory itself can become an important part of the dew-prevention system.
What Is Dew?
Dew forms when a surface becomes cold enough that water vapor in the surrounding air condenses on it. The temperature at which this happens is called the dew point. It is tempting to think that dew forms because the nighttime air temperature falls below the dew point. This is not the case. As temperature drops, the dew point normally changes along with it. The more important factor for astronomers is that your telescope doesn't necessarily remain at the temperature of the surrounding air.
Everything with a temperature above absolute zero radiates energy. On a clear night, your telescope radiates infrared energy toward the sky and receives comparatively little energy back. As a result, exposed surfaces can cool below the temperature of the surrounding air. This is why your telescope can be covered with dew even though the weather station says the air temperature is still several degrees above the dew point. The air may be 40°F and the dew point 36°F, for example, while the exposed corrector plate of your SCT has radiatively cooled to 35°F. The corrector plate is below the dew point, so water condenses on it.
A dew shield restricts the amount of sky visible to the objective or corrector plate, reducing its radiative heat loss. The glass stays closer to ambient temperature and takes longer to reach the dew point. A dew shield does not, however, prevent radiative cooling altogether. The glass still has a view of the sky through the open end of the shield. Given sufficiently unfavorable conditions and enough time, it can still cool below the dew point. A dew shield therefore delays the formation of dew; it does not guarantee that dew will never form.
A dew heater takes a different approach. Instead of merely reducing heat loss, it replaces some of the lost energy. Ideally, it supplies just enough heat to keep the optical surface slightly above the dew point.
What About Frost?
Frost is a closely related problem. In addition to the dew point, the air also has a frost point. The frost point is slightly higher than the conventional dew point at temperatures below freezing. At those temperatures, water vapor can deposit directly onto a cold surface as ice. For practical telescope operation, however, the basic problem remains the same: don’t allow an optical surface to radiatively cool to the temperature at which atmospheric moisture can accumulate on it.
It is possible for frost to form when the reported air temperature is above freezing. If the air temperature is 35°F, for example, an lens exposed to a clear night sky might radiatively cool below 32°F. If atmospheric moisture conditions permit, frost can then form on the glass even though the weather station never reports a freezing air temperature.
Dew Prevention in Your Backyard
For conventional backyard astrophotography, the traditional combination of a dew shield and dew heater makes very good sense. The telescope may be exposed to the sky continuously from dusk until dawn. If you’re asleep while an automated imaging sequence runs, nothing is going to intervene when conditions deteriorate. A dew shield reduces radiative cooling, allowing the glass to remain warmer for longer. A dew heater replaces enough lost heat to keep the glass above the dew or frost point after the shield alone is no longer sufficient. Using the two together means the heater doesn’t have to work as hard. This can matter because excessive heat around an optical system can have undesirable effects of its own.
Under those circumstances — unmonitored equipment that might be operating near the dew/frost point, our recommendation is straightforward: use a dew shield where practical and use a properly controlled dew heater.
A Remote Observatory Is Different
A well-managed remote observatory changes the assumptions behind that recommendation. The observatory continuously monitors environmental conditions and closes its roof when conditions become unsuitable for observing. Rain, excessive wind, clouds, and other unsafe conditions can trigger closure. Dew risk can be among those triggering factors.
Contrary to what some believe, the important measurement is not relative humidity by itself. It is the difference between the ambient temperature and the dew point — the temperature/dew-point spread. Operating in 80% relative humidity sounds alarming, but by itself it doesn’t tell us whether dew is imminent. What matters is how close the temperature is to the dew point and how much additional radiative cooling an exposed optical surface might experience.
To avoid the risk of dew formation everywhere on your equipment, not just on the glass, AstroPeak will close the roof when the ambient temperature is within 3-5°F of the dew/frost point. The exact margin appropriate for AstroPeak will be determined from experience and may change as we accumulate environmental data. The principle is more important than the particular number. Once the roof closes, something significant happens: the telescopes can no longer see the open night sky, and therefore don't radiate heat into it.
In that sense, the observatory roof is the ultimate dew shield.
An objective or corrector plate under the closed roof is surrounded primarily by the roof, walls, floor, telescopes, piers, and other objects that are at temperatures relatively close to ambient. Its radiative heat loss is therefore dramatically reduced. The glass tends back toward thermal equilibrium with its surroundings instead of continuing to cool through radiation to the open sky.
This gives a remote observatory a dew-control option that doesn’t exist for a telescope left outside all night: stop observing before dew becomes a problem.
Doesn't the Desert Climate Help?
AstroPeak is located in the high desert of southwestern Utah. The generally dry climate certainly works in our favor, but “desert” should not be confused with “dew is impossible.” The area around AstroPeak averages only about 11 inches of precipitation annually. But dry climates still have periods of increased atmospheric moisture, passing weather systems, winter conditions, and nights when temperature and dew point converge.
So we don’t propose relying on the fact that AstroPeak is in the desert. We rely on environmental monitoring and automated roof control.
Your Dew Mitigation Strategy at AstroPeak
Our first line of defense against dew and frost is not a dew shield or a heater. It is the observatory safety system. The observatory monitors temperature, dew point, precipitation, clouds, and other relevant conditions continuously. When the temperature/dew-point spread falls below a conservatively chosen limit, the roof closes. At subfreezing temperatures, the margin is adjusted to account for the slightly higher frost point. This changes how you should view conventional telescope dew-control equipment.
A dew shield remains useful. It reduces radiative cooling while the telescope is exposed to the sky and gives the system additional margin before the glass reaches the dew or frost point. If your telescope already has one and it causes no problems, there is no particular reason to remove it. But a dew shield can also wind area and swing diameter to a telescope. It can cause you to need a bigger (more expensive) pier. It can complicate the installation of automated flat panels and dust covers. Flexible shields can sag or interfere with equipment. At a remote observatory, those disadvantages may outweigh the additional protection a dew shield provides.
We therefore don’t consider a dew shield essential at AstroPeak if eliminating it simplifies an otherwise sound installation.
The same argument can be extended — somewhat more cautiously — to dew heaters. A properly controlled heater provides an additional layer of protection. An individual telescope might radiatively cool faster than anticipated, local conditions at a particular pier might differ somewhat from those measured by the observatory weather instruments, or glass might reach its dew or frost point shortly before the observatory reaches its roof-closing threshold. A heater provides insurance against those situations.
But consider what we’re insuring against. In a properly functioning observatory, we’re primarily trying to avoid losing the last few exposures before the roof closes. We are not depending on the heater to keep a telescope dry while it remains exposed to deteriorating conditions for the next six hours in your back yard.
For that reason, we continue to recommend dew heaters for primary optics that are susceptible to dew and frost, especially SCT corrector plates and refractor objectives. Dew heaters are inexpensive insurance, electrical power isn’t particularly scarce at a permanent observatory, and there is little reason not to use one when it can be installed conveniently. But that’s different from saying that a dew heater is inherently necessary for successful operation at AstroPeak. Under ideal operational practices, the telescope never reaches the point at which it needs one.
The goal of the observatory’s dew-control strategy is therefore not to make every telescope capable of surviving indefinitely under an open sky in increasingly unfavorable conditions. It is to recognize deteriorating conditions and close the roof while the equipment is still dry
Dew shields and heaters provide additional margin — the roof provides the solution.