I have a 250mm Mewlon. To solve a problem with condensation on the main mirror, I wrap a dew heater band around the back of the scope, near the main mirror cell. During the winter months I leave it on constantly at a low level, turning it off before observing. I have found this works well at preventing any moisture from forming on the optical surfaces or within the telescope tube. It only uses a tiny amount of power.
I also have a tube heater, something like 60W or 80W, set on a timer in the winter. This gives just a little heat to the observatory, but did not stop condensation within the optical tube.
I have a wooden roll off roof observatory.
Andy
British Astronomical Association
Supporting amateur astronomers since 1890