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I made a DIY dew heater for the secondary mirror on my 20" dob for less than $15 (assuming you have a power source already). It's dead easy and should be a transferable skill to smaller secondary mirrors. You need a USB powered heating pad (dirt cheap off eBay). Peel apart the two sheets of fabric and carefully remove the heating wire stuck to one face. I then used a bit of silicone to spot fix the wire around the back of the secondary (it's a big piece of glass), then run the wire along a spider vane, fixed with black electrical tape. They come with a heat controller, and even the lowest setting is enough to keep the mirror above ambient temp. Best DIY I've ever done (the bar was pretty low though).
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