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I wouldn't use a dew heater on a primary - as Scott said we usually are trying to bring DOWN the temperature of the primary to keep up with the dropping ambient.
Dew on the primary can be a problem in some circumstances when your scope is pointed almost straight up a lot, but a running fan will all but eliminate primary mirror dew problems. Combine that with a dew shield for the front end, and you'll fix most of your dew problems for good.
The secondary can still dew up on really wet nights, but a blast from a 12v hairdryer into the focuser will solve that.
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