Ground Telescopes

Very Large Telescope
The moon setting behind the Very Large Telescope in Chile

Most telescopes used by astronomers are on Earth. We call these ground-based telescopes. It is much easier and cheaper to build a telescope on Earth than in space. It is also much easier to fix if things go wrong. However, there are downsides as well. A telescope on the ground has to look through the Earth's atmosphere to see into space. This is a problem because the atmosphere can blur our images.

The air also blocks out light from parts of the electromagnetic spectrum. Parts like x-rays, gamma rays, infra-red and long radio waves. This means even if we have the right kind of telescope, it can't see this type light from Earth. The air gets in the way. This is why some telescopes are in space.

We call the parts of the light spectrum that can get through the air, atmospheric windows. These are the parts of the electromagnetic spectrum where the opacity (how much light is blocked) is close to 0%. If the opacity is 100%, then no light with that wavelength can get through the air to reach the ground.

Atmospheric Windows
The opacity of the Earth's atmosphere to light of different wavelengths.
Credit: NASA