As you may have guessed, astronomical telescopes are used to look at
things not on the Earth and terrestrial telescopes look at things on
the Earth. But there is a big difference in how they are made. To
understand this, I'll need to talk a little about how refractive
A refractive telescope (like astronomical and terrestrial) uses
lenses to bend light. Since stars or mountains that we want to look at
a long ways away, we want to gather as much light as we can from the
object and focus it all into our eye. To do this, there is a really big
lens in the front called an objective lens. The objective focuses the
light down to a very small area. Now, we can think of what we're
looking at as if it was in that little area that the objective focused
the light to.
Here's where the different telescopes come into play. An
astronomical telescope uses another, smaller lens to focus the light
from the small image into our eye. The problem with this is that the
image is upside down! A terrestrial telescope has another lens to turn
the upside down image right side up.
Now, you might think that it would be better to just use
terrestrial telescopes all the time. But that third lens has to be
placed away from the second lens. So terrestrial telescopes are a lot
longer than astronomical telescopes and therefore more expensive.
They each have their advantages. When we look at stars, we don't
really care that the image is upside down. And since the telescopes are
so big, it would be a lot more expensive to make the telescope bigger
just to see the image right sid up. On the other hand, when you are
looking at something on Earth, you want to see it right side up. But
since it doesn't need to be as powerful as the ones that look at the
sky, it is also a lot smaller and less expensive.
(republished on 07/29/06)