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Warm Eyepieces

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#26 dan_h

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Posted 18 January 2013 - 02:05 PM

I wonder if it would help putting a hand warmer on the eyepiece by using an elastic band to secure it on the eyepiece, (the side of course), ?? Hmmmmm....Has anybody tried this?


The chemical hand warmers do their best when in an enclosed space that will trap the heat such as a coat pocket or in an eyepiece case. Out in the open they don't hold up to the task very well. Having said that, I have used them inside a refractor dew shield by laying them along the lower edge and they did the job for me in a pinch. They will stop dew, they won't clear it once it starts. I have hand warmers under the eyepieces of a pair of binos and they were completely ineffective as the eyelenses fogged up as soon as I tried to look through them.

There are supplies of reuseable chemical heaters that put out a lot more heat than the little hand warmers. You can find some in a camping section of a sports store or in the baby section of some department stores. Boil the heaters to charge them. Shake them up to activate. You can get a wrap around model that is made to warm up baby bottles. (Should fit a 17mm T4 rather well.)

dan

#27 Sarkikos

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Posted 18 January 2013 - 02:07 PM

I haven't had much of a problem - if at all - with eyepieces fogging from my body heat or breath. That just doesn't happen for me. Maybe the key is to breath slowly but deeply when looking in an eyepiece? Don't huff and puff.

In general, I keep the eyecups on my eyepieces and try to add them to eyepieces that don't already have eyecups. Even for my little flat-top short focal-length eyepieces, I like to attach a shallow eyecup to help center my eye and shield it from ambient light.

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#28 Sarkikos

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Posted 18 January 2013 - 02:16 PM

Dan,

Having said that, I have used them [hand warmers] inside a refractor dew shield by laying them along the lower edge and they did the job for me in a pinch. They will stop dew, they won't clear it once it starts.


The best method to prevent dewing on a refractor objective is to make a longer dew shield for it. The dew shields supplied with most refractors are not long enough to keep dew from forming on the objective lens - at least in my area. Make them from black foam core, line them with ProtoStar or similar, and close it tight with Velcro strips. The ProtoStar or similar lining is important not only for glare prevention but to trap the dew and keep it from rolling down onto the objective.

I also make these for my binocular objectives. Easy to do and cheap.

When I have these home-made dew shields on a finder scope, refractor telescope or binoculars, dew will absolutely never form on the objectives, no matter how dewy the night is or how long the optics are exposed to the night air. Not gonna' happen'.

Mike

#29 Dick Jacobson

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Posted 18 January 2013 - 08:20 PM

I wonder if it would help putting a hand warmer on the eyepiece by using an elastic band to secure it on the eyepiece, (the side of course), ?? Hmmmmm....Has anybody tried this?

I have a friend who uses this method all the time. And this is Minnesota, so it should work anywhere.

I've often wondered if it would be practical to make an electrically heated focuser, or would the power demand be too great?

#30 Sarkikos

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Posted 18 January 2013 - 09:02 PM

We have electrically heated focusers now - you just have to wrap a dew strip around the focuser.

:grin:
Mike

#31 mich_al

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Posted 19 January 2013 - 09:38 AM

We have electrically heated focusers now - you just have to wrap a dew strip around the focuser.

:grin:
Mike


Tha's what I do but I question the sanity of it. The dew strip is a few Watts, wraped around an exposed piece of metal that is thermally coupled to an exposed OTA with enough surface area to radiate alot more than a few Watts.
How much of that energy goes to warm the eyepiece, even with a dew strip on the eyepiece. Seems like a loosing battle.

#32 Dick Jacobson

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Posted 19 January 2013 - 09:47 AM

We have electrically heated focusers now - you just have to wrap a dew strip around the focuser.

:grin:
Mike

Tha's what I do but I question the sanity of it. The dew strip is a few Watts, wraped around an exposed piece of metal that is thermally coupled to an exposed OTA with enough surface area to radiate alot more than a few Watts.
How much of that energy goes to warm the eyepiece, even with a dew strip on the eyepiece. Seems like a loosing battle.

A well designed electrically heated focuser would heat the drawtube but as little else as possible. Maybe there would be resistive heating elements embedded in the drawtube, and some kind of insulation between the drawtube and the focuser body. Sounds like an interesting project if there are any machinists out there reading this.

#33 dan_h

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Posted 19 January 2013 - 11:05 AM

We have electrically heated focusers now - you just have to wrap a dew strip around the focuser.

:grin:
Mike

Tha's what I do but I question the sanity of it. The dew strip is a few Watts, wraped around an exposed piece of metal that is thermally coupled to an exposed OTA with enough surface area to radiate alot more than a few Watts.
How much of that energy goes to warm the eyepiece, even with a dew strip on the eyepiece. Seems like a loosing battle.

A well designed electrically heated focuser would heat the drawtube but as little else as possible. Maybe there would be resistive heating elements embedded in the drawtube, and some kind of insulation between the drawtube and the focuser body. Sounds like an interesting project if there are any machinists out there reading this.


If you heat the focuser tube, the eyepiece will still dew up and that is the problem you want to fix. You will need to apply a lot of heat to the focuser to effectively prevent eyepiece dew. Sounds like a recipe to create thermal problems. This is a case where less is better.

Put a dew strap on the eyepiece as needed and be done with it.

dan

#34 Sarkikos

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Posted 19 January 2013 - 11:25 AM

Tha's what I do but I question the sanity of it. The dew strip is a few Watts, wraped around an exposed piece of metal that is thermally coupled to an exposed OTA with enough surface area to radiate alot more than a few Watts.
How much of that energy goes to warm the eyepiece, even with a dew strip on the eyepiece. Seems like a loosing battle.


Are we really worried about wasting a few watts of electricity? Are we going to having Green Telescopes now? I hope not. :foreheadslap:

I think convenience, adaptability and lowering initial costs are more important than wasting a few pennies in electricty. IME, my rechargeable 12v battery lasts at least eight hours. That's good enough for me.

Also, if we heat the focuser or other integral parts of the OTA internally, then we have a good chance of introducing thermals into the optical system. Definitely not a good thing. We should keep application of heat to the telescope to a minimum. I'll stick with the dew strips.

:grin:
Mike

#35 andyrud

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Posted 19 January 2013 - 01:09 PM

New guy here.

Could somebody explain what causes dew and also what is dew point?

Thanks in advance,
Andy R

#36 Starman1

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Posted 19 January 2013 - 02:32 PM

New guy here.

Could somebody explain what causes dew and also what is dew point?

Thanks in advance,
Andy R

Simple.
Cold air has less ability to contain water vapor than warm air.
So, as the night cools, eventually the water vapor content of the air becomes too high for the colder air to sustain and the water vapor condenses out on cold surfaces.
If the air is dry, that point could be at a very cold temperature. In fact, it may never get to that cold a temperature, hence, no dew. That is often the case in the desert or mountains in the Southwest.
If the air is moist, the condensation point might be reached with only a few degrees drop in air temperature.

Since horizontal surfaces exposed to the sky are radiating heat into space, and space is not radiating heat back, those surfaces become cold and it is on those surfaces dew first condenses.
Vertical surfaces radiate, most of the time, into the nearby environment which, in turn, is radiating heat right back. So those surfaces cool more slowly and are the last surfaces to get dew on them (in case you wondered why the top of your car got dew before the sides).

So, dewpoint is the temperature at which water vapor can no longer be carried by the air and below which the air must shed water. This is why the relative humidity rises at night, because humidity is related to the ability of the air to carry moisture. Hot air can carry more moisture, so if the absolute humidity (the amount of water vapor in the air) doesn't change, the relative humidity will be highest at night and lowest in the peak temperature of the daytime.

Hope I explained it well enough.

#37 andyrud

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Posted 19 January 2013 - 07:43 PM

Very good explanation Starman.

Thanks,
Andy R






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