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Cooling EP's?


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OK, we all know the benefit of cooling a scope before certain activities but is it beneficial to cool EP's too? Sometimes I wonder so leave an EP in the scope while cooling but with a cap on top so it doesn't dew......

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It's best to keep eyepieces slightly warmer than the outside temp actually. If they get too cold they tend to dew / mist from the warmth of your eyeball. Very frustrating.

Keeping them in a foam lined case or even in your pockets works.

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you won't get tube currents or a boundary layer on such a small area of glass so therefore cooling is not an issue with eyepieces. in fact, keeping them slightly above ambient temperature is good as this reduces dewing.

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Yeh i have often wondered on this one aswell, but like john says i often dew up my eyepices with my overly warm eyeballs :)

Likewise, and it is so irritating. However, the 12v hairdryer sorts it out :)

Dave

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Pockets are good for this. Make sure there isn't loads of lint and stuff in them though, pockets are dusty places!

I read somewhere that having waterproof eyepieces helps them to stop fogging up. I can't quite see the truth in this as it's a temperature differential that causes the fog up.

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Neither too warm or too cold. Dewing can happen when cold air comes into contact with a warm surface, or warm air with a cold surface especially if there are sudden changes.

Basically air can hold as much as 2% gaseous water vapour, 10-20mB in terms of partial pressure. Once it comes into contact with a surface be it cold or warm the air tries to equilibriate and it sheds water to do it. You see the effect on house windows when you have a warm indoors and cold air outdoors. Warm air can hold more moisture than cold.

The thing about relative humidity is that it is also affected by pressure in addition to temperature, that is why on one day it feels humid but on another day though the temperature is the same the pressure has changed and if feels dryer.

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Now this has me a tad confused. My 10mm doesn't seem to work unless it has been out of the box for 20 mins or so (outside). When I first went to use it, I thought it was faulty. Spent time with my 25 instead. Decided to drop the 10 in just as I was about to pack up, and whoa! It worked.

I asked on another thread about this but have not had a response yet.

This happens every time I view.

Is there a chance that this is mainly a problem on cheaper ep's (The 10 and 25 came with the scope)? Or is there a chance my 10mm just isn't up to scratch?

I'm still trying to decide on ep upgrades, but am looking very closely at a set of meade series uwa 5000 82 deg.

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EP's are a mass of glass and metal and as such it takes time for them to equilibriate with the ambient temps. If you have more glass (more powerful lenses) then if would take longer, glass is not a particularly good conductor of heat so it takes time.

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Saturation curve



Hopefully this will work and the attached image will show the effect of temperature and pressure changes in relation the saturation point of water vapour in air. Basically you need to stay to the right of the curve. Any time you cross that curve in any direction you change from gaseous phase to liquid phase water and hence get dewing.

I use this slide as part of a presentation I often do on dew point measurment which is one of my fields of work.
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The lower the barometric pressure the lower the saturation point as the maximum partial pressure of water vapour is lower, BUT, if the temperature is dropping then you can still hit dew point. That is the problem with Rh it is affected by both temperature and pressure.

If you wear glasses and you go from hot to cold or from cold to hot your glasses can mist up, that is dew point in action. Better still study the dew forming on a cool glass of wine or even a good glass of beer. Cold body, warm air....

However if the temperature of air is stable but the pressure is dropping so too does the dew point go lower. If the pressure remains the same and the temp increase then the dew point can rise.

Remember that the temperature I refer to can be the temperature of the mirror or lenses or eyepieces. They will lag any changes in ambient

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Well I use eyepieces with loads of big glass (ES 100, Naglers, Ethos, Pentax XW's etc) and I keep them either in the house or in their foam lined case when observing, ie: a bit warmer that outside temperature. I've only ever had dewing problems when the eyepieces have got too cold (ie: outside temperature). Otherwise I've had no problems whatsoever with eyepiece temperatures over the 30+ years I've been in the hobby. It's not scientific perhaps but it works for me :smiley:

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Thanks all. Seems keeping them in external pockets of a jacket for easy access is the way to go. I keep the rubber covers on to protect from lint, dew, and fingerprints etc. this gets them cooler than house temp but not so cold they dew up.

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I keep them warm so they don't fog up as easily if contacted by breath. When outside, they stay in an open case and do cool, but I still keep my most used eyepiece(lowest power) capped and in my coat pocket to keep it warmer.

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