Different Kinds of Traditional Reflecting Telescopes

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  • 0:03 Optical Telescopes
  • 0:44 Newtonian Telescope
  • 1:28 The Cassegrain Telescope
  • 2:22 The Schmidt-Cassegrain…
  • 3:22 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will discuss three major kinds of traditional reflecting telescopes and their major differences. These include the Newtonian, Cassegrain, and Schmidt-Cassegrain telescopes.

Optical Telescopes

The refracting and reflecting telescopes are the two major kinds of optical telescopes. Refracting telescopes use a convex lens to help produce images, while reflecting telescopes namely rely on a concave mirror.

Concavity is a shape that's like the side of a spoon which holds liquid, while convexity is the backside of the spoon, the one with the hump.

Although Newton wasn't the first person to invent a reflecting telescope (James Gregory was), his design became more popular and so he's often credited with inventing it. Newton's design, as well as two other designs of traditional reflecting telescopes, are covered in this lesson.

Newtonian Telescope

The Newtonian telescope is a kind of reflecting telescope that uses a diagonal mirror to reflect light out the side of the telescope. Thus, the eyepiece is positioned at the side of the telescope. This design is usually used in smaller telescopes.

Let's go over the diagram of a Newtonian telescope below to see what happens.

Newtonian Telescope
diagram of newtonian telescope

First, light enters from the left and hits the primary mirror at the back of the telescope. A focused image is then bounced off of a secondary mirror, through an eyepiece, and to the observer's eye. The secondary mirror is a type of mirror used in a reflecting telescope that directs light coming from the primary mirror to another focal point.

The Cassegrain Telescope

Larger reflecting telescopes use something known as a Cassegrain focus. A Cassegrain telescope is a kind of reflecting telescope where a secondary mirror reflects light back through a hole in the primary mirror's center.

Again, let's take a look at the image below to see what happens here.

Cassegrain Telescope
diagram of cassegrain telescope

First, light comes into the telescope from the left. It will, as usual, bounce off of the primary mirror. Then it will hit the secondary mirror, and from there it will travel through a central hole in the primary mirror where there's an eyepiece or a detector. The hole in the center of the primary mirror is called an aperture.

So, the big difference between the Cassegrain and Newtonian telescope should be quite obvious. The light rays are reflected to an eyepiece that's at the back of the telescope in the former instead of the side of the telescope as in the case of the latter.

The Schmidt-Cassegrain Telescope

A variation on the Cassegrain telescope, used in many small high-end telescopes, is known as the Schmidt-Cassegrain telescope. This is a reflecting telescope design that has a thin correcting lens at the front of the telescope tube in order to improve the image.

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