Hi, I use the following USB dew heater:
http://www.teleskop-express.de/shop/...cm-length.html
I originally bought it as it was reasonably priced and no controller needed. Plus I have USB on mount. Works well, but if anything it gets a bit too hot. I've been planning to write a review on it as I have a thermal imager and a couple of thermocouples. Imager shows there are some cold spots, but heat distribution is otherwise pretty good. When Run off 2.1 A USB it gets to 40 C or so, but off 1 A, around 30 C so there is a limited ability to regulate heat.
Still, I'm thinking about upgrading to a USB-focus Dewpoint controller when I upgrade my imaging scope.