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Dew shields

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alanbthomas's picture
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Dew shields

Having recently purchased a small (127mm) catadioptric scope, I was thinking of making an (unheated ) dew shield for it. I began to wonder, though, why wrapping a tube of material around the end of the OTA would stop dew forming. Damp air still has access to the lens. What does the shield actually do? In other words, what is the science behind dew shields? Any enlightenment gratefully received in advance.

Robin Leadbeater's picture
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Dew Shields

Dew forms on  a surface when the temperature of that surface falls below the dew point.  A  surface exposed to the clear night sky cools by radiating heat out into space which is only a few degrees K (This is an over simplification of course as the atmosphere blocks the radiation at some wavelengths but you get the idea).  A dew shield works by limiting the angle over which the heat can be radiated away. Cooling still takes place but more slowly.  There may be other effects. For example some people swear by rolled up newspaper or other absorbing material which perhaps preferentially absorbs the moisture in the air close to the optics.

Cheers

Robin

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Dew shields

Thanks Robin. That's interesting. Can we put any numbers on this? What is the optimum size of a dew shield? I'm playing devil's advocate, but that's part of the fun!

Robin Leadbeater's picture
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Length of dew shields

I've not done any scientific measurements but when I made mine I read that most dew shields are too made short and they should be 3x the aperture. This does look rather long though and gives  problems with the mount in a breeze.  

I first saw the newspaper trick on a small refractor at a sky party in Denmark. Most people who did not have heaters were struggling with dew but not this guy. (I think perhaps he changed it once during the night when it got too soggy)

Robin

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The effect of wind

As an aside,  dew is less of a problem in breezy conditions as the airflow is constantly removing the cooling boundary layer of air over the surface where dew is forming. (Which is just as well as my dew shield acts as a sail !)

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dew shields

Thanks Robin. I like the newspaper idea. It appeals to the improviser in me (not to mention the skinflint!).

A R Pratt's picture
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Dew shields

Hi Alan,

My dew shields are constructed from sheets of artists' black card from The Works arts and crafts supplies.

Clear skies,

     Alex.

alanbthomas's picture
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dew shields

Thanks, Alex. I may try that.

Alan

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Dew shields

As Robin says, a dew shield considerably reduces the amount of cold sky that the lens 'sees'. Without it the lens will quickly radiate it's heat to that cold sky and receive little heat from the sky in return. That loss in heat will cause the lens to go below the dew point of the surrounding air and water will then condense on the glass as a result- it will dew up. The night sky typically has a radiative temp in the UK of -10C to -30C. Last night, for example, I measured it as -26C.

The lens will try to reach radiative equilibrium with that cold sky but is prevented from going too low by the heat it receives by conduction from the bath of warmer air it is sitting in and the warmth it receives again by conduction from the scope tube etc. It is  a battle between radiative loss and conductive gain. That is why a breezy night where the transfer of heat from the air is more efficient the battle between radiative loss and conductive gain is more likely to be won. Also reduce the amount of cold sky you see (reduce the radiative losses) and that makes it more likely to stay dew free.

A dew cap does not work by trapping warm air as some books will say.

Cheers

Martin

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dew shields

Thanks for that clear explanation, Martin.

Alan

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My scope is mildly odd in

My scope is mildly odd in that the only optical surface exposed to the sky is a plane parallel disk of glass which serves two principal purposes.  One is to keep dust, wee beasties, etc out of the way.  The other is to support the Cassegrain secondary without introducing nasty diffraction spikes.

Dew is kept at bay by a very feeble heating element wrapped around the end of the tube and which doesn't appear to cause any annoying air currents.  Such a device could easily be used on any scope with an exposed front surface.