Dealing with Dew on a Humid Night

Summer in much of the world brings warm temperatures and often times brings nighttime dew. Dew accumulation can ruin a night of astrophotography by decreasing and diffusing the light that reaches the eyepiece or camera sensor.

To fight dew on a telescope, you can employ several methods and tools to minimize or eliminate its impact on your observing session. Here are some effective techniques:

Dew Shields: A dew shield is a sleeve that attaches to the objective end of the telescope and can be extended past the objective lens. It helps prevent dew formation by moving the lens farther away from the surface on which dew will form. Many quality telescopes come with a retractable dew shield that can be extended on a particularly humid night. It is helpful to monitor the local dew point, as it will give you an indicator that the dew shield should be extended when the temperature drops below this point. If your telescope does not have a built-in dew shield, there are after-market products available.

Eyepiece Warmers: Similar to dew heaters, eyepiece warmers are small heating bands or pads that you can wrap around the eyepieces. They keep the eyepieces warm and prevent dew from forming on their surfaces. The most common dew heaters have optional eyepiece warmers.

Dew Heaters: Dew heaters consist of various size powered straps that wrap around the telescope at the objective end. These straps are heated to a temperature above the dew point to keep dew from forming.

dew heater, dew controller, dew zapper
Thousand Oaks Dew Heater Controller

Fans: Fans, both internal and external, can keep the air around the objective lens moving. This can impede the formation of dew. If you use a portable fan, make sure it is not in direct contact with any equipment, as this will cause vibrations.

Anti-Dew Coatings: Some telescopes and eyepieces come with special anti-dew coatings. These coatings help repel moisture and delay the formation of dew. If your equipment doesn’t have these coatings, you can consider adding aftermarket coatings.

It’s best to keep an eye on the weather conditions, especially the expected dew point. If it is likely that the overnight temperature will approach or go below this point, prepare for anti-dew measures to prolong your imaging session. A dewy night does not need to make you pack up and go inside. By combining these strategies, you can significantly reduce or eliminate the impact of dew on your telescope, allowing for clearer and more enjoyable observing sessions.

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