Dew heater bands are used to prevent condensation on optical surfaces in cooler weather. The most common source of moisture is dew, however; moisture from your own eye can also condense on an eyepiece in extremely cold weather. When is comes to dew, the best defense is a good offense. Simply maintaining the temperature of your optical surfaces above the dew point will prevent condensation from happening. The most problematic optical surfaces, listed in order of their susceptibility are as follows:1) Corrector Plates on Schmidt-Cassegrain Telescopes, 2) Meniscus lenses on Maksutov Cassegrain Telescopes, 3) Objective lenses on Refractors, 4) Eyepiece eye lenses in very cold weather. Thousand Oaks heater bands are wrapped around the outside of the area to be heated, such as around the metal cell of a corrector plate or objective lens. They attach themselves using a built in Velcro elastic strip. In the case of an eyepiece, the band wraps around the eyepiece itself. Very little heat is actually required, and too much heat can have negative effects as well. These heater bands feature standard RCA jacks to receive a 12 VDC power supply. Use of an adjustable dew heater controller is highly recommended.