This page is under development and so will be modified from time to time as the theory and practice relating to thermal problems becomes clearer. My attempt is to provide herewith a detailed discussion of thermal problems affecting telescopic viewing and imaging and related problems of dewing of equipment due to climatic temperature and humidity conditions.
It is usual to hear observers say that the telescope should be kept at ambient air temperature or nearly so. Still because of dewing problems on some of the optical surfaces or other surfaces, heaters of various sorts are applied. Sometimes heaters are applied to the optical tube and to the eyepiece as well. So what is going on here? Why does dew form? What other effects does the application of heaters have on the optical system? What is the effect of wind or artificial air movement? The undesirable effects of temperature and temperature differences seem to be well known but an understanding of how the physical mechanisms work seems to be limited to engineers who have understood thermodynamics.
It is best to separate the thermal effects into three parts. One is the effect of non uniform temperature on the optical characteristics of the telescope itself, another the effect on the optical path of differential air temperatures and the third the dewing results which are temperature related.
For a variety of reasons, it is necessary to force the telescope optical system to go to the ambient air temperature. This is because the telescope is a large object and exposed to the ambient air and it would be impossible to insulate it from ambient temperature for any length of time anyway. Normally, the ambient temperature changes over a night of viewing by many degrees. The actual ranges depend, of course, mainly on the climate at the observing local. One can imagine changes of tens of degrees from sunset to dawn. Unfortunately, such temperature changes will cause a number of optical effects in the telescope optical system. With a well designed telescope, the collimation should not change, but it has been shown that the focus does change. This change is probably detectable for even a degree or so temperature change. Complex folded telescopes seem particularly sensitive to temperature. There is not much that can be done about this except to watch for focus changes over the night and re-focus regularly.
One of the most obvious optical effects is the visibility of air waves rising from objects that are warmer than the surrounding ambient air. (similar waves are visible from objects that are cooler than ambient). This is probably the main reason for keeping the telescope at the same temperature as the ambient. Similarly, when the telescope is in a building and must look over the walls of the building or through an open shutter in the dome of the observatory, there might be non- uniform temperatures that cause heat waves in the air. These heat waves, whatever their origin, cause the image formed by the telescope to waver or scintillate. While the eye can account for some scintillation, in imaging the result is often serious fuzzing of the image. There are modern digitally controlled means for reducing some of this scintillation. For example, the AO-7 unit available from SBIG has been reported to reduce scintillation considerably.
But, clearly, avoiding optical path problems by reducing or eliminating the effect in the first place would be a wise design criterion. It is probably easiest to eliminate heat waves caused by the observatory building by removing the building completely with a roll off structure. This can be done by rolling the entire building away from the telescope pier or by opening the entire roof of the building. This factor is probably one of the strongest for using such a roll off design for the building. Additionally, the roll off structure is relatively inexpensive and simple to construct.
However, it should also be possible to create a dome/shutter design that avoids heat waves in the vicinity of the shutter opening as well. I believe that this can be done by ensuring that the dome is brought quickly to ambient temperature and kept there with fans that move ambient air through the building. The air should be drawn in through the shutter and vented at the side of the dome away from the shutter. It is probable that several fans will be needed to bring the dome and its support structure to ambient in a convenient length of time. Faster moving air cools the building and telescope quite rapidly.
There are conflicting requirements for reducing the problem of heat waves in the optical path. As seen here, the roll off building seems to have an advantage. Still there are several other considerations that will demonstrate an advantage for the dome/shutter configuration.
When looking at all of the vagaries of dewing, the results seem to be confusing and the methods for reducing dewing a black art. Dewing is not, in the simplest situation, a complex phenomenon. Simply stated, moisture condenses on a surface when that surface is even slightly below the dew point of the air. This temperature is quite well defined and can be measured to a small fraction of a degree. When the air is moving, dewing becomes more complex. Air motion will directly affect the thin layer of stagnant air near the surface within which dew formation takes place. Moving air also strongly affects the heat transfer between the air and the surface.
Generally, the surface temperature on which dew forms must be below that of the air for dewing to take place. In situations where there is a large amount of moisture in the air, the dew point temperature may be only a few degrees below the ambient air temperature. I such cases, dew forms on surfaces that are cooler than ambient temperature very easily. In many situations, when there is considerable moisture in the air during the daytime, the ambient air temperature and the dew point approach each other as the nigh temperature falls. In the clearest nights, the temperature tends to fall fastest.
Accepting these basic concepts, it is clear that an essential part of the dewing mechanism is the actual temperature of the surface on which we hope the dew will not form. So it is time to consider the equilibrium temperature that a surface will take when exposed to ambient air and its surroundings. The equilibrium temperature of a surface will be established when the heat energy reaching and leaving the surface are the same. There are three mechanisms for heat transfer. Conduction from a warmer or cooler body, convection of heat to or from the surrounding air and radiation of heat to and from the surface. Let us take these in order.
Conduction of heat to a surface in the case of a telescope is typically from the attached equipment. This might be the electronics and/or motors driving the telescope. Or, very typically with amateur telescopes, heat is applied from a so called dew zapper. The dew zapper is a heating element whose purpose is to raise the temperature of the telescope slightly. Generally it is applied at the mirror or corrector plate since that surface seems the most subject to dewing. The amount of heat has to be held to the absolute minimum necessary to stop dewing, and no more, since there are other deleterious effects of heating and raising the temperature of the telescope above the ambient air temperature. Many amateurs find this equipment not only effective but necessary, especially when the telescope is used in the open or in a roll off building where it is exposed to the cold night sky.
Convection is the transfer of heat from the air to the surface or vice versa. It is normally necessary to have the telescope very near the ambient air temperature to prevent convection currents from occurring and causing heat waves near the telescope objective/corrector plate. In order to increase the heat transfer from the air to the surface, the air must be moved. With good air motion, the telescope will move rapidly toward the ambient air temperature. This is good for several reasons. One is the reduction of heat waves just mentioned. Another is the reduction of the possibility of dew formation. If the surface is at the ambient temperature of the air, dew cannot form. (unless the dew point temperature is actually equal to the air temperature)
Radiation is a much more subtle way to transfer heat. When two bodies are at different temperatures, there will be heat transfer from the warmer body to the cooler body. The transfer is dependent upon the temperature of the bodies and the area that each body sees of the other. (It also depends on the nature of the surfaces in a complex way, but this effect will be overlooked in this discussion.) In the case of a telescope which is in an open setting, the telescope sees the entire sky and radiates heat to the entire sky. The effective radiation temperature difference between the telescope and the sky is quite large. The sky (atmosphere) has a temperature of about 200 Kelvin. The telescope, even at say 50 F, has a temperature of 10 Celsius, of 283 Kelvin. Thus the telescope will radiate more heat to the sky than it receives and it will cool off. If it cools just a few degrees, it might well drop below the dew point temperature. Then dew forms. All surfaces experience this same effect. We all know that any surfaces on an open viewing situation, plastic cases, chairs, wooden boxes, telescopes, everything gets wet with dew when dewing conditions are right. In fact, they get wetter and wetter and water will run and puddle on them.
There is a constant struggle between heat radiation from the surface to the sky and heat transfer from the air to the surface. The temperature of the surface will and must be below the ambient by some amount. Thus the surface can dew up. A dew shield on the front of the telescope will greatly reduce the tendency of the corrector plate to dew up because it greatly reduces the solid angle of the sky that the corrector plate sees. However, eventually the corrector plate will dew up if the heat transfer from the ambient air cannot keep up with the radiative cooling to the sky.
A simple demonstration of this effect can be made by placing two similar surfaces out in an open field. Over one of them, by a couple of feet, place a sheet or blanket. The exposed surface will dew up easily and the protected one much later. This is because the one is protected from seeing the cold night sky.
Now what has this to do with observatory design? One strong reason to use a the domed observatory is to prevent the telescope from seeing the open sky. Inside the observatory the air and the dome itself will be very close to ambient air temperature if proper air motion is provided. (forced if necessary) Thus objects inside the observatory including the telescope and other equipment see a "sky" which is the inside of the dome which is nearly at ambient. (Note that the dome will be radiating to the open sky and will thus be slightly cooler than ambient.) Thus there is no net radiation from the objects and they will remain at ambient temperature and will not dew up. The same effect helps keep the observer warmer since the observer is not radiating to the cold sky but to the dome which is at ambient temperature.
This is a very strong argument for a properly designed dome type observatory. The natural or forced flow of air through the shutter helps keep the telescope at ambient air temperature as well. Telescopes used in domed observatories rarely have dewing problems.
The above discussion is somewhat simplified, since the nature of the surfaces comes into play, the amount motion of the ambient air is important and other non-linear effects take place. However, I believe the first order effects are well taken into account.
Return to Beginning
Go to Home Index for Doc G's Info Site