AstroSystems amazing Dew Guard secondary mirror heating system automatically senses the air temperature and the mirror temperature, then maintains the secondary at a preset (adjustable) temperature above the ambient air temperature. Gently warms the secondary at 2-4 watts (depending on size) when on full and has 4, 6 or 8 evenly spaced contact points for optimal thermal distribution. Uses any external 12 volt dc supply or you can run on a 9v battery in a pinch. No more heat ropes!