Celestron Thermistor DewHeater Control.

Thermal switch for heating controller.
Take the guesswork out of using
Third-party heating tapes and increase your energy efficiency
with the Celestron thermistor for intelligent DewHeater controls.
This accessory allows the Smart DewHeater Controller (separate
available), just the right amount of heat for the corrector of your
to deliver telescopes without wasting the battery.
How it works:
Connect the heating tape to your telescope and place the
sealed thermistor probe tip under the heating tape.
Plug each cable into its corresponding port on the Smart
DewHeater control.
NOTE: The Celestron Dew Heater Ring does not have this switch
needed. It already has a built-in thermistor.