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Have you tried a littlehotties hand warmer stuck in a sock around the focuser. That should give you 8 to 18hrs of heat depending on the size. I use them on my lenses when imaging all the time. Great passive heating. No power requirements. You can order a pack of 80 on eBay for $40 or there about or get them from Costco fro $20 for 40 I think.
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