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Binoculars and Dew

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#1 ZachK

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Posted 19 June 2006 - 01:45 AM

I setup my binoculars last night (Celestron 15x70 skymaster on a Bogen tripod). And went back inside for a bit, when I came back out they optics were covered in Dew. I've read that dew shields will help somewhat, but I'm wondering if there are any other tricks for getting rid of dew once it starts to build up. I checked and my wife does not have a hair dryer. I live in the Judean desert so I expect dew is going to be a pretty constant thing over the summer, twas very frustrating as it would have been quite a nice night to observe otherwise.

#2 btschumy

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Posted 19 June 2006 - 07:18 AM

When you are not using the binoculars, keep them pointed downward so the lenses are not exposed to the sky. This will slow the radiative cooling that leads to dew formation. You could also keep the lens caps on when not actually observing.

If the dew is still forming then another strategy is to make some dew shields using rolled paper, thin cardboard, or thin closed cell foam.

If even this fails then you will need to look into some sort of electrical dew buster. Google for "Kendrick dew" and you can read about one such system. There are do-it-yourself plans on the web as well.

#3 dgs©

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Posted 19 June 2006 - 08:58 AM

I wouldn't have thought your humidity was high enough to cause dew problems. Is the humidity high often?
I assume you know that dew forms when surfaces cool to temperatures at or below ambient dewpoint temperature (saturation temperature for air at local barometric pressure). If the air is relatively dry, a surface would have to get quite cold to condense moisture out of the air.

At any rate. Bill has the right answers. Keep optics covered when not in use. If the problem persists, try simple home-made dew shields of thin black foam or even stiff black felt. These will block the 'view' of space that your optics have from off axis (outside the Field Of View), which will minimize the radiant heat transfer from your lenses to space. They also do a good job of blocking stray light from entering the optic path, thereby potentially enhancing contrast.

If that still isn't enough, you will need to resort to electric heating.

As high as my humidity is (often in the 80% range or higher) I seldom have dew problems on the objective end (my binocular sessions are not prolonged), but the eye lens (and telescope eyepieces) will collect moisture quickly as sweat evaporates from around my eye (possibly some moisture from the eye itself too) and condenses on the eye lens. I have taken to keeping a rag in my pocket to wipe my face periodically, while I fan the eyepiece with my hand.
I have read suggestions on here to use a squeeze bottle for ketchup or mustard, etc. to puff on the lens.

At any rate, keep at it and let us know how you beat this problem. ;)

#4 Glassthrower

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Posted 19 June 2006 - 10:13 AM

I've used a variety of anti-dew strategies, and the best solution for objectives is a simple dew shield as mentioned above. Cut some wide strips of black construction paper and put some velcro strips on the ends (available at most arts-and-crafts stores and office supply stores) ... roll the strip around the objective cell and affix with velcro. It will stay in place, forestall the formation of dew, and block off-axis light. When not in use, the shields lay flat for storage.

I am currently in the process of researching/making some dew heaters for my eyepieces that will be powered by a 12V battery. I'll post the results when I am done.

Good luck and clear dark skies...

MikeG

#5 ZachK

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Posted 19 June 2006 - 09:55 PM

I wouldn't have thought your humidity was high enough to cause dew problems. Is the humidity high often?
I assume you know that dew forms when surfaces cool to temperatures at or below ambient dewpoint temperature (saturation temperature for air at local barometric pressure). If the air is relatively dry, a surface would have to get quite cold to condense moisture out of the air.


Its the desert, its hot and dry during the day, but then cools off a LOT at night. By 11 pm its quite cool and the same about of mositure is in the air, so its very dry at noon when its 85F out, but by 11pm dew becomes an issue.

I was out last night. Just on my back porch. It was quite nice for about an hour. I scanned much of Scorpius and Sagitarius. Saw some really nice star fields and open clusters. I was totally unable to see M4 despite the fact that I was looking right at antaries.

Oh well I'll try it again tonight.

#6 DJB

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Posted 20 June 2006 - 03:39 AM

Hi all,

Good advice here. I use homemade and purchased dew shields, when necessary. Also I ALWAYS keep the lens caps on tightly until ready for use, e.g., as the binocular is cooling down.

Also, I've noted, that on a cold winter's night, I have the most problem with the eyepieces. I can only assume that the transference of heat from my face (body) to the eye lens drives the dewpoint condensation equation crazy, back and forth, back and forth, ad infinitum. Good luck.

Best regards,
Dave.


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