I used to use one of these with the 300mm lens
http://www.oceancontrols.com.au/CET-005.html
The hysteresis is configurable or the thermostat on off interval. It kept the lens within about 1.0C of set temperature.
But now I use this PID controller with five dew straps
http://www.tetech.com/Temperature-Co.../TC-48-20.html
It keeps the temperature of the RH200 within +-0.1C of set temperature. Friday night it was just below zero C in the observatory and the controller was at 40% power or about 40w and the RH200 was at 17.0C. This of course was distributed over five dew straps. These dew straps would be big enough for a ten or twelve inch scope.
Bert