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Depends a little on how heavy your dew is.
Up to about 3 inch diameter glass, telephoto or refractor, i have found the cheap USB / 2W heaters work perfectly well in moderately heavy dew. In really heavy dew, just give up and go to bed. I bought a pack of 3 in 2015 and they are still all working.
One got snagged once and the pull caused all the heating elements to bunch up. Amazing it didn't rip the electrical leads out. It still keeps the dew off. I just put the heaters at the bottom so the heat rises across the lens surface. I mainly use it for a finder scope now and use the others for the imaging optics.
Joe
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