Why is the eyepiece on the side of some telescopes?
If you are a beginner to telescopes and shopping around for scopes you may be wondering why the eyepiece is on the side of some telescopes rather than at the bottom end which seems to make more sense.
There are many different types of telescopes and some have lenses while others have mirrors.
Telescopes that have the eyepiece on the side are of the reflecting kind. This means that they have a mirror at the bottom that reflects light back up the tube. There is also a second mirror that reflects the light out of the side of the tube where the eyepiece is.
This next illustration shows a common refractor telescope. The light from the sky comes into the tube from on the right at the big lens. It is bent toward the small lens where you put your eye. This is the type of telescope that most people think of when they think "telescope".
But a reflector telescope has a mirror that bounces the light and bends it. In the picture below you see the light come into the tube on the left, it bounces off the mirror back up the tube to a secondary mirror that bounces it out the side of the tube where the eyepiece is. Thus the eyepiece is at the top and the side of the tube. There are some advantages to this configuration because it often makes for more comfortable viewing. If you used a refractor telescope pointed straight up you would probably have to lay on the ground to look in the eyepiece but with a reflector it is in a nice comfortable position.