Although operating the scope inside an observatory is very helpful, dew can still form on the optics when it is very humid and the temperature drops. Dew forms when the temperature falls below the "dew point" - the temperature at which the relative humidity increases to 100%. [Note: It is probably well-appreciated by readers of this web site, but I'll mention it anyway: 100% Relative Humidity does NOT mean that the air is 100% water (!), but that it is 100% "saturated" - i.e., holding as much water as it can at that temperature and pressure.] Telescopes are aimed at the sky, so the optics are radiating their heat energy out to space. Most everything else is also radiating heat, but also absorbing heat (e.g., from the ground); however, the optics uniquely lose heat but do not gain heat from their surroundings - which is space (!).

In order to minimize dewing of the telescope optics (especially the front corrector plate of the LX200 catadioptric scope), dew heater strips are placed around the optics. For example, I use a heater strip around the corrector plate, another for the Telrad, another for the Meade 8x50 finder scope, and still another if I'm using an eyepiece. These dew heaters are typically just resistive elements (possibly nichrome or another material that heats up when electric current is passed through it) that are typically connected to a 12V power source.

However, it is undesirable to heat the dew strip to high temperatures, as it might cause convection currents or other distortions of the optics or atmosphere in or in front of the scope - which could affect an image. Simple dew heater controllers use potentiometers to adjust the voltage provided to the dew heater, thereby controlling the heat output.

The Kendrick Premiere Dew Controller has microprocessor-controlled functions, including controlling up to 6 different heater strips, each at fixed temperature, or certain temperature above the dew point. Therefore, as the dew point changes, the heaters can adjust to keep the optics just above the dew point - and thereby prevent dewing - with just enough heat, and not so much as to affect the image.

The Kendrick dew controller system can be connected via a serial (to USB) port to the observatory computer, and entirely controlled remotely. Optec provided a 3-headed through the [Paramount ME] mount cable that incorporates the Pyxis, TCF-S, and Kendrick dew control serial ports.

In the above image, you can see the dew heater strips around the front of the LX200 (slightly angled dark band), and around the front of the Meade 8x50 finderscope (smaller blue scope in picture above). The cables from all of the dew heater strips are connected to the controller using standard RCA phono plugs. The Kendrick controller also has a temperature/humidity probe, which is the short black thingy sticking out the upper left side of the dew controller.