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Dew Shield- just a cozy?

JCJC Registered Users Posts: 768 Major grins
edited December 19, 2011 in Accessories
I frequently leave my camera out overnight for astrophotography. In the summer. In the desert. No problems.

I want to a make a dew shield for my camera for colder and more humid conditions. My camera and some of my lenses are semi-sealed, and I don't leave it out if If theres a real chance of rain, all I want to do is cut down on condensation.

Basically, all I need is a sleeve or a cozy for the lens that will hold a heater pack of some kind against the lens, right? I'd probably use one of the iron filings heating pads, or a reusable on of about the same size.

Theoretically, my sensor (filter) would also be staring at a black body, but I figure the sensor keeps itself warm by running, so I don't really need to heat the body up, unless I'm worried about battery performance.

Anything I'm missing?
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    IcebearIcebear Registered Users Posts: 4,015 Major grins
    edited December 15, 2011
    I would do some investigation into whether the heat packs give off any kind of gasses as they oxidize that might condense on your lens glass. I've wondered about this in the past when I've used them to keep a point-and-shoot warm when winter climbing. Never really scientifically
    looked into it though.
    John :
    Natural selection is responsible for every living thing that exists.
    D3s, D500, D5300, and way more glass than the wife knows about.
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    ziggy53ziggy53 Super Moderators Posts: 23,921 moderator
    edited December 15, 2011
    I agree that you should not use a chemical heater/hand warmer*.

    If you must do this, use an electrical heater. To be ideal, you just need enough heat to prevent the "localized" humidity from reaching 95%, or so.

    There are commercial products that can be adapted for this purpose. A heat band used for preventing the same condensation problem from occurring in telescope optics is ideal for the lens:

    http://www.telescopes.com/telescope-accessories/dew-prevention/dewheaters.cfm
    http://www.amazon.com/AstroZap-Heater-Bands-Telescopes-Size/dp/B0045IZBG6/ref=sr_1_24?s=electronics&ie=UTF8&qid=1323952656&sr=1-24
    http://www.amazon.com/Thousand-Oaks-Four-Channel-Digital-Control/dp/B0033UKAFO

    You can use a hermetic (or close to hermetic) box enclosure for the camera, so that it doesn't need to be heated. After use, just keep everything in a closed bag until the temperature equalizes to room temperature before opening.

    *(The problem with iron-based heaters is that they almost always use salt (NaCl) as part of the formulation. As the iron + water + salt reacts, it produces heat, but it also produces salt-rich water vapor. Using one of these warmer packs close to your equipment will almost certainly induce corrosive elements into your system, likely causing both mechanical and electrical corrosive consequences.)
    ziggy53
    Moderator of the Cameras and Accessories forums
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    JCJC Registered Users Posts: 768 Major grins
    edited December 19, 2011
    ziggy53 wrote: »
    I agree that you should not use a chemical heater/hand warmer*.

    If you must do this, use an electrical heater. To be ideal, you just need enough heat to prevent the "localized" humidity from reaching 95%, or so.


    *(The problem with iron-based heaters is that they almost always use salt (NaCl) as part of the formulation. As the iron + water + salt reacts, it produces heat, but it also produces salt-rich water vapor. Using one of these warmer packs close to your equipment will almost certainly induce corrosive elements into your system, likely causing both mechanical and electrical corrosive consequences.)

    Thanks Ziggy, I was wondering if the vermiculite would offgas, I wasn't thinking about the salt. I do have a sodium acetate re-usable hand warmer I could use. I just can't add another battery powered thing to my field gear, especially a 12 volt. I've already got AAs for p&s, GPS and backup headlamp, AAA for main headlamp, back up flashlight, C-something or another for main flashlight, lithium batteries for SPOT satellite messenger, and two different canon proprietary batteries, oh, and laptop batteries. Add a 12 volt to that mix, and trying to keep it straight which ones are charged and which ones aren't, I think my head would explode.

    So, I'm thinking a well insulated pouch for the sodium acetate warmer to keep it working longer at a lower heating rate, and some sort of cozy to hold it close to the lens. Maybe wool felt.
    Yeah, if you recognize the avatar, new user name.
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    ziggy53ziggy53 Super Moderators Posts: 23,921 moderator
    edited December 19, 2011
    You can try the sodium acetate approach, but I think you'll find the heat production the opposite of what you need, i.e. it will produce large amounts of heat rather quickly in the crystallization process and then taper off over time.

    Ref this paper for specifics, especially the chart on page 26:

    http://www.motiva.fi/files/929/master-s-thesis-mikko-keinanen.pdf

    Surrounding the sodium acetate package with a super-insulator might give somewhat desirable results, but you would still require a fairly large mass of sodium acetate to begin with.
    ziggy53
    Moderator of the Cameras and Accessories forums
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