How To 

How to Prevent Your Telescope from Fogging Up


Telescopes are amazing pieces of technology that allow us to glimpse parts of the universe we would otherwise not be able to see clearly. But, just like any piece of tech, there are some downsides that need to be addressed. One of the biggest problems amateur astronomers have with telescopes is dew and fog. Here’s how to prevent moisture on your telescope from ruining your view.


1. Don’t keep your telescope in a warm environment.
This sounds wrong, doesn’t it? Shouldn’t you protect your telescope at all costs and keep it in a safe, temperature stable environment? Not necessarily. Temperature difference is what causes dew in the first place. If your telescope is warm and the air outside is cold, the moisture in the air is going to condense on your telescope, leading to dew and poor seeing. By ensuring your telescope isn’t significantly warmer than the ambient air, you reduce the chances of dew forming.

2. Add a dew shield.
Newtonian telescopes rarely have dew problems because the optics are way down in the tube. Other telescopes don’t have deep tubes, so what you can do is extend the tube and provide your optics with additional protection. If you keep the air in front of the optics at a stable temperature by shielding them with a dew shield, dew won’t form.


3. Try a dew heater or a heated dew shield.
The best way to combat dew is to use double prevention. A dew heater keeps your telescope at a stable temperature, while the shield aspect extends the tube. Dew heater strips are options as well, but they don’t offer the added protection provided by a shield.
Very good and easy to read and follow. Sometimes I leave my telescopes in the garage that is outdoor air before using. Then I take them out for a view. If you setup near a river, a cool night can quickly bring dew everywhere :)

Dwight Huth

Oct 22, 2019
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I used a 1 gallon bucket to make my dew and light shroud.

I simply traced the outline of the telescopes aperture on the bottom of the bucket, cut the area out using a Dremel. To help create a tighter seal around the optical tube itself, I first cut an old computer power cable into circumference sections the same as the circumference as the optical near the aperture. I then used Gorilla Glue to glue the cords in place. To keep the plastic bucket from scratching the tube, I used Gorilla Tape and wrapped the bottom of the bucket. I then spray painted the entire bucket a flat black to keep light from reflecting off of the shroud and fouling the image being taken.

The Bucket Shroud works great for keeping dew off of the front lens. I can go for maybe three hours without even having to check for dew compared to having to check and wipe the front lens every half hour.

The Bucket Shroud also helps keep light behind me and from 180 degrees at the front of the lens from creating additional light pollution.

A few photos.