Types of Telescopes

Types of amateur telescopes: Refractor, reflector, catadioptric 

Let's look at the three major types of amateur telescopes. Refractors, reflectors, and compound – or catadioptric – telescopes. When most people think “Telescope” they probably imagine a refractor, like the one here at the top of this page. They have a large glass lens at one end, through which light passes and refracts – or bends – the light to a focal point. As the light passes the focal point, we place an eyepiece, which then focuses the light into a magnified image we can see. Often, a mirror or prism called a “star diagonal” is placed here to bounce the light 90 degrees for more comfortable viewing.

At right: A 90mm f/10 refractor telescope on a small equatorial mount.














So far so good, right? A reflector, on the other hand, is often called a Newtonian telescope. These can be mounted on equatorial or Dobsonian mounts. And yes, this type of telescope was named after Isaac Newton, who first came up with this telescope concept. As the name implies, light is reflected off of a mirror in this kind of telescope, usually placed at the bottom of a tube. As it is reflected, it is towards a focal point as well, but note where that happens: Near the top of the tube! That makes it hard to place an eyepiece there, as both the focuser and your head would block incoming light. So a reflector uses a “Secondary mirror” - a small, diagonally-shaped flat mirror to bounce the light out the side of the tube, like this, where it enters the focuser and reaches the eyepiece.

At right: A 10" reflector telescope on a Dobsonian style mount.













The third type of telescope is a compound telescope, or catadioptric. “Cat-a-what?” I know, that's a big word: It means, “an optical system that involves both the reflecting and refracting of light, in order to reduce aberration.” So this Schmidt-Cassegrain telescope, for example, utilizes a “Corrector lens” on the front of the telescope, a strongly curved primary mirror at the back, and then another outwardly curved mirror in the middle of the corrector lens before light finally goes through the center of the back of the telescope to the eyepiece. It basically can take a very long focal-length light path, and “fold” it into a shorter tube. There are other versions of catadioptric scopes, Schmidt-Cassegrains and Maksutov-Cassegrains being the most prominent amateur types available.

At right: An 8" Schmidt-Cassegrain style catadioptric telescope on a fork-style, equatorial mount.









Let's take a quick overview of the strengths and weaknesses of each:

Refractors

For refractors, here are some advantages: 

  • Unobstructed design and ease of manufacture allows for theoretically better optical quality

  • Eyepiece location is convenient in smaller models

  • Tend to be portable in apertures of 100 mm or less and are easy to aim

Some disadvantages include

  • Chromatic abberation in achromats, where not all wavelenghts of light focus in the same place

  • Apochromatic designs, though overcoming chromatic aberration, are usually 3 to 5 times as expensive

  • In longer focal lengths, the long tubes can sway in wind or on inexpensive mounts

  • Larger models place eyepiece uncomfortably low

Reflectors

Reflectors have their own set of advantages: 

  • No chromatic aberration

  • Easier and less expensive to make in larger apertures

  • Faster focal ratio systems provide wide fields of view

  • Light grasp in larger models is excellent

  • Cost per inch of aperture is best

But they also have their drawbacks:

  • The optical aberration “coma” occurs in faster models, unless specialized eyepieces or correctors are used

  • Light loss due to multiple mirrors is greater than refractors

  • Central obstruction due to secondary mirror can cause diffraction and contrast loss

  • Larger models can be bulky and heavy

  • Eyepiece can be in awkward positions

Catadioptric / compound telescopes

Catadioptrics, likewise, have pluses and minuses. On the plus side, they  

  • Have reduce optical aberrations to minimal levels

  • Provide good light gathering power

  • Eyepiece generally in very convenient location

  • Offer excellent portability

On the downsides, cats are

  • The most expensive per inch of aperture

  • Have the greatest light loss due to multiple lenses and mirrors

  • Often have the largest central obsctruction

  • Tube currents due to sealed systems can cause poor images before fully cooled to outdoor temperatures

So that is a quick look at the three major types of telescopes, and some of their better features and downsides.