They are usually "dumb" heaters... literally just some voltage applied to wire (you can read more about them here, along with calculation according to wire AWG

They are usually "dumb" heaters... literally just some voltage applied to wire (you can read more about them here, along with calculation according to wire AWG and length: http://www.blackwaterskies.co.uk/2013/05/making-your-own-nichrome-dew-heater.html http://www.blackwaterskies.co.uk/2013/05/making-your-own-nic...) A major pain point of dew-heaters is if they are too strong, they induce "tube currents". E.g. the heat from the dew-heater causes the air around the heater to start convecting, which causes ripples in the final image. It's not really visible as ripples, but a general blurring of the image. It's essentially recreating astronomical seeing conditions inside the telescope itself, which is obviously not ideal :) I think some fancier setups will be "active" and regulate themselves based on ambient temperature, but it's a lot more common for people to over-build their heaters and then use a dimmer switch. So you make your dew-heater for the worst winter night you expect, then dim it down for the rest of the year manually.