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There are probably three tactics you can use against dew.
Shield the objective... so you could make a longer dew shield to extend the tube further out and this will shield the objective from more of the sky so it doesn't lose so much radiant heat and it helps to keep the air new the objective there and at the same temperature.
Use a dew heater. This would require a substantial battery and preferably a controller for your dew heater. Have a look in the Projects and Articles section. I've written a couple of articles about building your own dew heater controller and making your own dew heaters from resistors if you want to save a few dollars and DIY.
The last tactic I can think of is to use a 12V hair dryer (and of course a battery) You would only use the hair dryer to clear the lens once it has fogged before you need to use it. This is probably not the most efficient way to go, but some of us have to use all three of the methods!
Al.
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