Silly question about dew.

[SmokeyJoe]

Hi, this will be my first winter of observing so please forgive my silly questions.

The past couple of nights my scope has been wringing wet by about midnight.

I live in little village in Norfolk surrounded by fields if this makes any difference.

What conditions cause dew?

Is this going to happen every night over winter?

In short, should I be asking Santa for an anti dew system rather than a nice new ep?

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[RogerTheDodger]

http://en.wikipedia.org/wiki/Dew_point

[JamesF]

A few shield is probably the place to start, and see how you get on with that. As long as you can keep the optics dry you should be fine I think. Sometimes you find that most of the dew has settled by the time you go out and sometimes it just keeps falling out of the sky all night long. I've come in on a few evenings when whichever refractor I've been using has been completely encrusted with frozen dew

How bad the dew is depends both on what temperature you're starting from and how much it falls, so it can vary greatly from day to day.

James

[Bizibilder]

You can make your own dew shield quite cheaply out of a camping mat (Argos or Tesco sell them) - just make a tube that slips over the end of the scope. With a reflector it is dew on the secondary that is more likely to cause problems. The primary has, in effect, the whole tube acting as a dew shield.

[Moonshane]

as long as it's left to dry naturally and not sealed inside the telescope dew, with the caveat below, will not harm your scope in any way so don't worry about it.

dew on the primary can (over a long period) affect coatings as it dissolves some of the chemicals in the air and on the surface of the mirror (unavoidable) but a mirror with dew on it constantly will deteriorate far more quickly.

I store my scopes in the house and in the winter even when the tube itself is dry, the mirror steams over when brought into the house. I leave my caps off and the scope set at about 45 degrees to prevent drips falling on the mirror.

not something to worry about and a dew shield or dew heaters will not stop the scope getting soaking wet. you can prevent dew on mirrors with a dew shield but newts are no too bad for that. I use a home made dew shield and have never felt the need for a heater.

[sam mayo]

i had terrible dew problem with my250px when i first had it then i made a camping matt dew shield but still found the secondry a bit of a problem so i mad a mini dew shield for the focuser and now i dont get any problem or not any worth worrying about

[acey]

Dew is caused by exposure to the clear sky (leading to heat radiating from the object and reaching a lower temperature than the air adjoining it). Reduce exposure and you reduce dewing. A dewshield does this. Don't leave your optics exposed to the sky if you're not looking through them: lower your scope or cap the end, keep eyepeices capped or in a case when not in use. The rate at which an object dews depends on its emissivity: glass dews quickly, metal more slowly, wood even more slowly. A wet telescope or mount is no big deal - just give it a wipe afterwards. Dewed optics are the issue, and if shielding and other measures are not sufficient then use blown air (e.g. a 12v hairdryer) or dewstrips. Dewing is most likely on still nights and is unlikely if there's a breeze (which has the same effect as a hair-dryer, constantly repleneshing air on the optics). A dewshield should be as long as possible: 1.5 times aperture is a good minimum. It can be made of anything that wil lretain shape and not get soggy (thermal properties are completely irrelevant). Camping-mat foam is popular because it's cheap and easily cut - for small apertures it may be too thick to be convenient, in which case cardboard works fine (I use cardboard for my finder dewshield).

[SmokeyJoe]

Thanks for the info. I have a newt and am more worried about damage to the scope. I shall start taking precautions.

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