Dealing with dew
2021 February 2
The problem of the formation of dew on astronomical equipment is one familiar to all observers in our famously damp islands. We seem to be constantly fighting condensation and spending money on methods of combatting it. It is perhaps worst at this time of year, but it afflicts us to some extent in all seasons.
Physics of condensation
Dew formation results from the properties of the mixture of gases making up the air. Air cannot contain much water vapour. How much depends critically on the temperature.
The psychrometric chart shows the moisture content of air at different humidities and temperatures. We see that for a given moisture content (horizontal line on the graph), reducing temperature increases humidity, to the limit of 100%, or when saturated. At this point precipitation, or condensation, occurs on any solid surfaces. In clouds and fog this occurs on dust grains, but even if there is no fog, condensation will occur where surfaces of objects that are slightly colder than the air push it to the point of saturation at the interface. This is where the dew that covers our equipment (and the grass, parked cars, etc.) comes from.
But why is our equipment colder than the air? The answer is the radiative transfer of heat. A cloudless sky has a radiative temperature of –20 to –100°C. Objects facing the cold night sky will try to reach radiative equilibrium with it, and cool down as a result. In still air, objects can cool to several degrees below the air temperature. Dark and transparent objects cool faster (hence the term ‘black-body radiation’), while reflective objects such as polished metal or aluminised plastic cool more slowly.
In the UK humidities of 50% or above, and temperatures between 5 and 15°C, are normal in the daytime. As the psychrometric chart shows, saturation occurs as this air falls to 0–5°C, which regularly occurs at night. Moreover, after a couple of hours under a clear sky, surfaces will commonly be 1–2°C cooler than the ambient air. Under these conditions, dewing is inevitable.
The worst conditions typically are encountered on nights of good transparency, after the passing of a cold front, when an absence of high water vapour makes the sky colder. If rain has occurred earlier in the day, then humidity close to the ground will be very high. If there is little wind then there is little warming conduction to surfaces, hence they cool faster. All these factors together often give rise to mist and fog, but heavy dewing will occur first.
The first line of defence against dew is to try to keep the temperature of our equipment from falling so fast. This can to some extent be done by shielding it with another surface that is not as cold as the sky. The object glass of a refractor, corrector plate of a catadioptric telescope or lens of a camera is highly exposed, but placing a long dew shield over it greatly reduces the solid angle of sky that it ‘sees’, and slows cooling, delaying the onset of dew. With solid-tube Newtonians the tube itself acts as a long dew shield for the objective, but the secondary mirror can often dew up.
Durable dew shields are best made of aluminium or carbon fibre, but in an emergency, basic improvisation can do the trick. On an observing trip in Wales once, I was plagued with dew on my camera lens, but I found that by fitting it with a shield made from the hotel brochure and Sellotape, I could keep it off for long enough to take some photos.
While clarity of optics is our overriding concern, water on telescope tubes and mountings is not helpful to their long-term performance. Hence a major benefit of a domed observatory with a narrow slit: all the equipment, not just the objective, is largely shielded from the sky and kept above the dewing temperature. Short of this, benefits can be got by shielding the external surfaces of a telescope, or cladding a truss or skeleton-tube telescope with aluminium foil or reflective Mylar film.
Shielding only delays the onset of dew on bad nights, but it may be prevented entirely by heating the components to keep them above the dew point. Blasting optics with a hairdryer for a few seconds is an emergency measure, but it will distort the optics, temporarily creating aberrations or loss of focus. Over-heating may also cause undesirable local air currents that give the effect of bad atmospheric seeing. So it is much better to apply very slow, gentle, even heating all around the optics with a dew heater band. With good shielding it is only necessary to heat them to a fraction of a degree above the air temperature to prevent dewing; such heating will not cause detectable aberrations or seeing effects.
Heater bands are manufactured in many different sizes. They have resistive heater wires enclosed in a fabric covering and are fed from a 12V DC supply. While you can connect them directly to a battery or mains power supply unit, it is better to limit the energy input to them with a dew heater controller, to give only the minimum heating necessary. Controllers simply act as timed switches, turning the current on and off periodically. You can experiment with them to find the minimum power setting that keeps dew at bay.
An interesting case was reported to me by member Trevor Smith in Derbyshire. He was finding that the object glass of his 4-inch refractor, fitted with a dew shield, was regularly dewing on the inside. The telescope was stored in an unheated garage. I recommended he store it with a small canister or bag of dry silica gel desiccant in the drawtube, to dehumidify the air inside. For this to work it would be necessary for the tube to be well-sealed. Cooled CCD and CMOS cameras often have desiccant containers inside them as well; else the cooling might cause condensation within the camera.
David Arditti, Director, Equipment & Techniques Section