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Old 10-04-2009, 07:43 PM
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erick (Eric)
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Camera lens - dew

Is there any reason why I shouldn't wrap a dew heater around the lens of a camera to keep it clear all the time it is pointing at the sky. I'm thinking of several hours timelapse shots.

I cannot think of one. However, get it to temperature first, then set focus, is the only thing I can think of.
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Old 11-04-2009, 04:41 AM
Dennis
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Hi Eric

I have found that wrapping a dew strap around the lens prevented dew forming and I was able to image all night long. I would still use the lens hood for the lens, to also prevent stray light from striking the optics.

I have used the dew straps on lenses from 28mm wide angle to 400mm telephoto.

Cheers

Dennis
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Old 11-04-2009, 01:53 PM
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[1ponders] (Paul)
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It's alwasy a part of my setup Eric, 3" or 4" dew strap to the thousand oaks controller. works a treat.
17-40mm
50mm
85mm
135mm
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Old 11-04-2009, 10:46 PM
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erick (Eric)
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Thanks gentlemen, I'll keep the lenses warm this way.
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Old 12-04-2009, 09:37 AM
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Omaroo (Chris Malikoff)
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Yep Eric - I use a lens hood (different ones, of course, depending on the lens) and heat this. Works a treat. Keeps out stray light and adds a dew shield.
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Old 12-04-2009, 09:41 AM
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erick (Eric)
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Heat the lens hood, Chris? That is interesting, I'll try that - keeps the dew heater away from the focus/zoom/aperture ring.
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Old 17-04-2009, 05:32 AM
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montewilson (Monte)
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This will only work if heat gets from the lens hood to the glass of the lens. If not your hood will be as dry at the Sahara but your coke bottle will dew up in now time.

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Originally Posted by erick View Post
Heat the lens hood, Chris? That is interesting, I'll try that - keeps the dew heater away from the focus/zoom/aperture ring.
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Old 17-04-2009, 06:29 AM
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Quote:
Originally Posted by montewilson View Post
This will only work if heat gets from the lens hood to the glass of the lens. If not your hood will be as dry at the Sahara but your coke bottle will dew up in now time.
It seems to work beautifully Monte as it is. Heavy dew last night and the lens/filter was clear-as all evening. The old debate rages on - do we heat the optical surface itself or the air immediately in front of it? There are arguments for both - but my arrangement has always worked well, so I leave it. I guess I'm still not a fan of heating the objectives themselves...

Last edited by Omaroo; 17-04-2009 at 06:46 AM.
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Old 17-04-2009, 11:51 AM
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That's leaking heat into the glass of the lens. The thread of the hood will easily pass it through the metal and into the glass. It still works OK but has nothing to do with heating the air near the glass. That's my story and I'm sticking to it!
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Old 17-04-2009, 12:40 PM
cruiser (Brett)
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Hi guys,

newb here regarding dew heaters. I plan to get a dew strap for my 100ed as well as one to use around a camera lens. Question: If advised to let equipment stand a while to get to correct temps etc. doesn't a dew/heater strap counteract some of the cooling that may have taken place or as its only heating the outer shield / glass element then it doesn't affect internal temps? Hope this makes sense and doesn't sound too stupid.

Brett
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Old 17-04-2009, 08:19 PM
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Hi,

Sorry, another question regarding dew heaters. I've recently cooled my 350D using a water cooled Peltier which showed great improvements in the dark shots taken. I then tried taking some shots at 0C through my ED80 and found it worked well for about an half and hour before my Hutech LPS filter (which was screwed to the end of my 2" primary focus adapter) decided to ice over while inside the ED80 tube

After reading this thread I was wondering if wrapping the ED80 focus tube with a dew heater might fix this problem? I don't have one to test so any suggestions would be helpful.

Thanks,

Markus

Last edited by KISA; 18-04-2009 at 12:36 AM.
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Old 18-04-2009, 07:37 AM
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Hi Markus & Brett - You can heat up the tube, but if the tube is getting so cold then that heat will eventually find it's way into the camera, then the heater and the Peltier will be in a fight to the death. You will need to find a happy medium.

I would need to see some pictures of the set up to comment more.

On the subject of letting the equipment cool down this is more of an issue for reflectors and not much of an issue for cameras and lenses. I wouldn't worry about it unless you are talking about a temp difference of more 20 deg C. Even then it will cool down without heating in minutes after which you can start warming the lens. Don't forget heating is a misnomer. It should be thought of as nothing more than gentle warming. If the ambient temp is 5 deg C and your lenses are at 6 deg C you wont get dew.

Another helpful trick is to cover the lens with an old sock (you can use a new one but you'll get in trouble if you have a sock manager like I do). Cut a hole in the end so the lens has a woolly sleeping bag. This reduces heat loss to the air and will mean you need less heating power too which is a big help with batteries. Camera lenses are usually the first to dew up because they have the least thermal inertia so a warming solution is a must.
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Old 18-04-2009, 09:22 AM
KISA (Markus)
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Thanks Monte. I agree there probably would be a conflict of temperatures. I'm wondering if it's possible to put a dew strap a certain distance up the focuser shaft so the heat would travel down enough to warm the LPS filter (and stop the dew) but only minimally effect the CMOS which doesn’t directly touch the tube and sits a couple of inches further down? I've never used a dew strap (or any dew heater for that matter) so I don't really know anything about them. Are the dew straps temperature variable? Where can I buy them or is it possible to make them? They sound like they make life a lot easier and save the neighbours from the sound of a hair dryer at strange hours in the night
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Old 18-04-2009, 02:25 PM
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Make one from Nichrome wire. Connect about 1m of the wire to 12v and feel the wire. Slide one end of the power wires along the Nichrome until the wire is just warm. That should be enough. I wrapped my scope tube in tape them wound the wire around the tube and then covered the lot with more tape and finally some neoprene (any thin flexible foam will do) you will need stuff all power to keep the tube warm.

I know this description is pretty vague so let me know if this makes no sense.
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Old 18-04-2009, 04:02 PM
KISA (Markus)
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It makes total sense, I'll chase down some Nichrome wire and give it a go.

Thanks again,

Markus
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Old 26-04-2009, 08:59 PM
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you guys are going to think I'm crazy...but it works...and I don't know why...I took a piece of black construction paper and wrapped the corrector of my C6 (making a 8" dewshield basically) after it had completely dewed over...within 10 minutes it was dew free...I had two witnesses that couldn't believe it either...prior to putting it on they both told me I was "done for the night"...I have since ordered a Kendrick Digicap but will always keep a homemade in the "might need it kit"...also at 99 cents a sheet its a very cheap alternative to have around for those "oh crap I forgot my dewshield" moments...
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Old 27-04-2009, 06:07 AM
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This is quite believable. By adding the shield you reduced the rate of heat loss enough for the warmth in the body of the scope to pass through to the glass of the corrector and warm it up . Without additional heating the corrector would have eventually dewed up later in the night but what you tell us is a great example of the effectiveness of a simple dew shield.
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Old 27-04-2009, 08:23 AM
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Alright Monte - gloves off! LOL!!

The only reason that I can see to heat the glass itself would be to raise the temperature of the air right at the glass/air interface - such that it's dewpoint is raised. Dew condenses out of air, not glass. Therefore - heating the tube with the intent of heating the glass, in turn, services this requirement because you are radiating heat from the glass into the adjacent air - as does heating the air at the bottom of the dew shield anyway like I do. Same thing - only one is possibly slightly more efficient that the other in terms of the volume of air that we are heating. A dew shield provides an air trap, of sorts, in order to keep some of the previously warmer air (from the earlier part of evening) close to the glass so that it doesn't approach the dewpoint as quickly as air further up near the open end of the dewshield, which is at ambient temp and probably falling. The differential need only be a couple of degrees. I wouldn't mind engaging in a conversation with a qualified thermodynamicist to see what the crux of the matter really is. I'll get back...

At the end of the day - I guess I'd rather heat air than potentially alter/deform the refractive qualities of a finely ground and collimated fluorite objective by heating it directly. Hmm... the debate rages on... LOL!

Last edited by Omaroo; 27-04-2009 at 08:49 AM.
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Old 28-04-2009, 04:34 PM
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Hi Chris - try this, heat your lens hood but not the lens. Put the hood very close to threading in with maybe a 3 or 4mm gap. Cover this section with sticky tape or cling wrap so no air can pass through the gap. Point the camera up at the night sky with the heater on. My theory is you will have dewing as fast as you would have if the heater weren't on at all. My thinking is there will be no heat transfer from the hood to the lens body. If you are right the warm air will keep the lens from dewing. The heat should be as much you would normally use. You could heat the lens with IR from the hood if you had it hot enough.

I don't have a dew strip to try it so you will have to be the myth buster if this thread is ever going to be resolved (in my favour).
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Old 29-04-2009, 06:20 PM
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multiweb (Marc)
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Quote:
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At the end of the day - I guess I'd rather heat air than potentially alter/deform the refractive qualities of a finely ground and collimated fluorite objective by heating it directly. Hmm... the debate rages on... LOL!
Sorry Chris I have to strongly agree with Monte on this one. You're right to say the water vapor is in the air but it will only condense onto a surface that is colder than the dew point. If you keep the metal holder of your lenses warm just above it then you have no problem. Regardless of the air in front of the glass surface. How do you keep the air just in front of your lens warm enough when there's a breeze anyway. Besides any metal/glass exposed to the night sky will "freeze" well below ambient air temperature even if you've got a air dryer handy. It won't last long. Nichrome wire cut to size wound up around the tube is the only way to go. Gentle heat transfers through metal to glass.
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