Refracting & Reflecting Telescopes: Key Concepts

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  • 0:03 Reflection and Refraction
  • 0:39 Primary Lenses and Mirrors
  • 1:27 Focal Length and Focal Point
  • 2:10 Chromatic Aberration
  • 3:17 Reflecting vs…
  • 4:18 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will discuss some key terms and concepts relating to the refracting and reflecting telescope: primary mirror/lens, eyepiece, focal length, focal point, chromatic aberration, and achromatic lens.

Reflection and Refraction

Did you ever point a laser at a mirror and watch it bounce off? Did you ever notice how if you stuck a pencil straight into some water, it would appear to bend? You know it's not really bent, but it looks that way.

These two simple examples demonstrate the properties of reflection and refraction, respectively. The bouncing and bending of light are two ways light can be manipulated for use in two different types of telescopes, known as the refracting and reflecting telescopes. This lesson will discuss some key concepts related to their use and why one type is the leading kind of telescope used today.

Primary Lenses and Mirrors

Another lesson introduced you to the reflecting and refracting telescopes. In summary, a refracting telescope uses a lens, sort of like those in magnifying glasses, and a reflecting telescope uses a mirror. There are also the catadioptric telescopes, which use a combination of lenses and mirrors.

The main lens used in a refracting telescope is known as the primary lens and, easily enough, the main mirror used in a reflecting telescope is called a primary mirror. In either case, both telescopes will make a small and inverted image of whatever it's pointed at.

Since you kind of need to see what in the world you just pointed the telescope at, you rely on the lens nearest the eye, the eyepiece, which allows you to see the image.

Focal Length and Focal Point

The eyepiece has a short focal length. A focal length is the distance between a lens or mirror to its focal point. And the focal point is the area where parallel beams of light that strike a mirror or lens are brought to a focus thereafter. Those definitions aren't easy, so it's best to just show you an image of what I mean.

Diagram of focal length and focal point

There you can see parallel beams of light going through a lens and then converging to a central focal point. The distance between the lens and focal point is the focal length. Mirrors and lenses that have a short focal length have to be strongly curved, while long focal length lenses and mirrors, less so.

Chromatic Aberration

You are now ready to understand one of the limitations of refracting telescopes. As light is refracted through a piece of glass, the shorter and longer wavelengths of light don't behave the same way. Short wavelengths of light will bend more and come to a focus closer to the lens than longer wavelengths of light. That is to say, blue will have a shorter focal length than red.

This means that if you focus your eyepiece onto a blue part of an image, all the other colors will be blurry. The same goes for focusing the eyepiece on a red part of an image; all the other colors will also be blurry.

This distortion is termed chromatic aberration. Something called an achromatic lens can be used to better the situation but not by too much. An achromatic lens is a telescope lens that is made from two separate lenses, which are in turn made of two different types of glass, that will bring two different wavelengths of light into the same focus.

Now, while this helps, it doesn't solve the problem. While two colors are brought together, the other colors will still be out of focus.

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