I just got the latest edition of Australian Sky and Telescope. On page 82 is a half page article on "Preventing Dew Problems". I am sceptical about the explanation of how a dew shield works. Let me explain...
As a professional mechanical engineer I have studied thermodynamics and physics at undergraduate level, and applied same to numerous problems in twenty something years of engineering practice.
The explanation for how a dew shield works, according to the article, is due to radiation loss to the "chill of space". A dew shield reduces exposure to the sky and so reduces heat loss due to radiation. My initial reaction was "No way!". BTW I have seen this explanation before on the web, but passed it off as just mis-information.
So I did a little bit of searching on the net, only to find this seems to be widely accepted by the astronomical fraternity!
I am prepared to concede I can be wrong. But the more I look at the problem the more I doubt this explanation.
Firstly, the lens is not exposed directly to space. It is exposed to the atmosphere which in turn is exposed to space. Heat loss via radiation from the atmosphere to space is not disputed here!
...but to calculate the heat loss from the lens to space it is necessary to analyse a radiation network (in the case of radiation losses) and indeed a full heat transfer network. BUt just to keep it simple, lets just consider radiation for now...
All matter emits radiation. The frequencies of the radiation is determined by the material, and the intensity of the various frequencies is determined by the temperature. Radiation heat transfer equations are based on the difference of temperature between two materials, and other factors for emissivities, absorbtivities, and turbidity of the intervening space, etc.
The radiation network from lens to space is a triangle. Some of the radiation leaving the lens goes to space and some goes to the atmosphere (keeping this simple). The third side of the triangle is radiation from the atmosphere to space. As the temperature difference between the atmosphere and the lens is minimal, there is minimal radiation loss if any.
But the atmosphere contains carbon dioxide and water vapour (also known as green house gases) and these absorb most long wave infrared radiation such as would be emitted from our lens. So while the path from our lens to the sky is optically clear, at infra-red frequencies generated by materials at normal ambient temperatures, it is not. So the large majority of the radiation from the lens goes to the atmosphere, not directly to space, and since the atmosphere is at a very similar temperature, the net radiation loss is insignificant.
Now it's all very well to write this and pick the "radiation loss" theory to bits, but let me follow it up by explaining how I believe a dew shield works (I'm prepared to believe it up until it's proven otherwise!
).
The dew shield works by two mechanisms. Firstly it effectively stops the movement of air across the face of the lens. In other words it traps a pocket of air in front of the lens and stops it from moving too much. As the dew point in the air is reached, the water vapour in the air immediately adjacent the lens condenses on the lens but initially this is not visually detectible. The rate of condennsation and evaporation at the lens reaches an equilibrium following Le Chatelier's principle. With the air trapped in fromnt of the lens the rate of transfer of water vapour to or away from the lens is limited to diffusion, slowing the rate of condensation on the lens, becasue as the temperature drops further, water vapour diffuses out into the moving air stream at the end of the dew shield or it supersaturates until a dew droplet is formed in the air (which then falls down).
So the first mechanism the dew shield uses is to reduce the rate of dew formation on the lens by limiting air movement.
The second mechanism is to act as an umbrella. When air cools below the dew point, dew will condense on anything it can. It it can't condense, on a solid like the lens it will eventually condense on dust in the air, etc forming dew droplets. Once formed, these rapidly grow in size and settle due to gravity. So unless the scope is pointing up steeply, the dew shield will catch a lot of the falling dew and so shield the lens.
So that's my explanation of how a dew shield works, but what's more I propose an experiment and some measurements that can be done to see what's what...
If the radiation loss theory is dominant, the lens should be significantly colder than the surroundings. Some references I have seen suggest this is the case, so I suggest this should be measurable. I don't have the means to take these measurements accurately enough, unfortunately, but I propose another experiment that could be easier to manage.
The experiment goes like this: set up a series of glass plates exposed to the night sky.
a) One exposed to the night sky with no dew shield or cover.
b) One horizontal with a dew shield, and
c) One vertical with a dew shield.
d) One horizontal with a cover the same distance away from the glass as the end of the dew shields, but sized to shield the same proportion of sky as a dew shield, and
e) a similar arrangement but horizontal.
If the radiation loss mechanism is dominant, the unshielded glass will dew up sooner and more heavily than the others.
If my theory of how it works is correct, c) and d) will dew more slowly / less than the others.
Now I really doubt that I'll get around to doing the experiment, but I'd like to. If anyone knows of sound scientific analysis or experimental results that "prove" radiation loss is the way a dew shield works, I'd really like to see it. At the moment, I believe it falls into the category of an "astro" myth...
I hope I have got you thinking about it! Such a humble piece of equipment, the dew shield...
Al.
...but I've enjoyed the mental exercise of working through the problem!
