MadSci Network: Astronomy
Query:

Re: How do I prevent my telescope lence from fogying?

Date: Sat Jul 1 12:07:38 2000
Posted By: Neil Sandham, Secondary School Teacher, Science Teacher and Career and Technology Studies Coordinator, Chestermere Middle School
Area of science: Astronomy
ID: 961957285.As
Message:

I probably don't have to tell you this, but for others who may read this, the reason your lens is fogging up at night is due to condensation. Water vapour in the warm air touches the colder lens and (according to the Particle Theory of Matter) the water particles lose enough energy that they change from a gaseous state to a liquid one on the lens.

There are two easy ways to solve this problem, as long as the lens is only fogging up on the outside.

Some advanced astronomers have told me that they wipe their lenses with silica gel, like the kind that comes packaged with running shoes. This keeps the outer lenses dry by absorbing moisture before it has a chance to condens on the lens.

Unfortunately, if the inside of your lens is fogging up, it is because there has been moisture trapped between the interior lenses. Many telescopes have sealed cases which prevent this, but once the case has been cracked or opened, moisture gets in between the lenses. In this case, I don't know of any easy way to keep them from fogging up. The advanced astronomers I contacted fill their telescope casings with nitrogen gas, which pushes out water vapor and (as long as the seal is kept) prevents internal fogging. However, most of these astronomers use telescopes with mirrors and refractive lenses. These are a little more expensive (but easier to make yourself) and don't tend to fog up as much.


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