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Condensation trouble
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Norbs
Joined: 09 Aug 2006
Posts: 5
Location: Cheshire, UK
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 Posted: Thu Aug 24, 2006 11:34 pm Post subject: Condensation trouble |
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Hi 
Its been an especially clear night tonight here in the North West, England. Quite chilly though my fingures are just about working the keyboard 
The big problem I had tonight was condensation on my lenses. The worst was my finderscope. My secondary mirror and eyepiece were also bad. Is there anything I can do to stop this.
Anyone got any tips  |
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bauzl
Joined: 22 Aug 2006
Posts: 6
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| Posted: Mon Aug 28, 2006 7:29 pm Post subject: |
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Hi Norb,
usually condensation occurs while the air is cooling down and can no longer hold the same amount of water like warm air. At a certain low temperature there is no help about it and the air becomes foggy (-> dew point).
But there is an additional problem with the optical surfaces and there might be some help:
The surfaces are exposed to feel the cold breath of space, that is the temperature of the radiation from space is only around 4 degrees (Kelvin that is, very near to absolute zero). So the surfaces tend to radiate their termal energy out to space and become even cooler than the surrounding air - and condensation starts there first.
Here in the alps everybody knows, that the windshield of a car can be covered with a thin layer of ice in the morning, even if the air temperature never dropped below freezing. (That happens in all the months containing an "r" in their name, unless its really cold and you have snow as well  )
Usually our friends from the refractoring liga (see below) use dew caps on their refractors, that is, lets say an extension tube in front of the real tube. This limits the amount of sky seen by the objective lense - and therefore limits the amount of energy lost through radiation. They need to do this, because the objective lense realy sits on top of there instruments. You could try to do this with your finderscope. Make it just wide enough so that it does not reduce your field of view.
As for the primary mirror in a Dobsonian, it already has its dew cap - the Dobsonian's tube. I also see little chance to improve the situation for the secondary, as it is not directed towards the sky.
Some people use some kind of electric heating (simply resitors) for mirrors and lenses to keep them just above the dew point avoiding thermal air currents caused by too strong heating - I never tried. That's too complicated for me, and so far I didn't need it.
I hope I could give you at least some hints
Clear skies - and mirrors
Bauzl
PS: O.K., even if I loose a Dobsonian point now: I also have a small refractor 
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Norbs
Joined: 09 Aug 2006
Posts: 5
Location: Cheshire, UK
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| Posted: Tue Aug 29, 2006 11:19 am Post subject: |
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Hi Bauzl,
Thanks for the advice 
The primary mirror didn't dew up until I brought the scope inside, so I left the cap off to let it breathe. I guess that this is because of the large difference in temperature that the tube can't counteract.
Do you think it will be better to lift my scope out during the day so it cools down gradually?
My main concern is that my lenses will be damaged from the dew.
I also have a small refractor  Perhaps we should start a 'Small Refractor Anonymous' group  . I haven't used it very much, it is very old, probably antique. |
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bauzl
Joined: 22 Aug 2006
Posts: 6
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| Posted: Wed Aug 30, 2006 6:39 pm Post subject: |
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Hi Norb,
I also use to leave the cap off until the mirror has dried.
If you have a closed tube I guess you can simply put the cap on outside and don't remove it inside until the scope and the air inside is warm again. As the mirror didn'r show any condensation outside, the same air, when it gets warmer will become even dryer, that is it can hold more water. So the mirror will not suffer.
best regards
Bauzl
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