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How Telescopes Form Images

Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

A telescope is an optical tool that forms images of distant objects that makes them appear closer than they are. Learn how telescopes form images using the objective and focal plane, the eyepiece lens, reflecting and magnifying telescopes, and how telescopic sight compares to human eyesight. Updated: 10/28/2021

Eyes, Telescopes, and Images

From the outside, a person's eyes are just a couple of golf-ball sized things stuck inside their head. But if you've ever taken an eye apart in a classroom, you clearly would've seen the very complex nature of this amazing natural apparatus. It's got a lens, iris, retina, fluidy stuff inside, and way more than that. And under the microscope, it gets even more complex!

A telescope is pretty similar. A telescope you can get at the local store is just a tube to me, but like our eyes, it's made up of many parts.

While we've got an anatomy and physiology course that covers how our eyes detect light, this lesson's big mystery and focus lies in figuring out how a tube somehow takes light coming from a distant object and then produces an image your eyes can then appreciate. We'll end this lesson with a cool comparison of the light-gathering abilities of your eyes compared to that of telescopes.

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  • 0:02 Eyes, Telescopes, and Images
  • 0:59 The Objective and Focal Plane
  • 2:00 The Eyepiece Lens
  • 2:59 Reflecting Telescope
  • 3:38 Comparing Everything…
  • 5:56 Lesson Summary
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The Objective and Focal Plane

A reflecting telescope, aka reflector, or a refracting telescope, aka refractor, both use something called an objective, a lens or mirror that focuses light. Since telescopes are all about great visuals, let's walk through this lesson with lots of images, otherwise what I'm about to say won't make too much sense.

For simplicity's sake, let's start off this lesson by saying our telescope is using a large diameter lens, the objective lens. The larger the diameter of the objective, the greater the light-gathering power of the telescope.

Once light coming from our celestial object enters the telescope, it will hit the objective inside the telescope. The objective will then form an image in something known as the focal plane, the plane where the objective of a telescope forms (or focuses) an image. The distance from a lens or mirror to its focal point is called the focal length of that lens or mirror.

The Eyepiece Lens

But we're not done yet! While our large diameter, long focal lens at the front of our telescope forms the image, we need something else to help us to actually be able to see the image!

That's where a smaller lens, the eyepiece lens, located at the back of the telescope comes in. You can see how this lens has a much shorter focal length than that of the objective lens:

Diagram of Telescope
diagram of lenses of a telescope

This is important! That's because the magnification of the image made by the objective is equal to the focal length of the objective lens divided by the focal length of the eyepiece lens. The smaller the latter, the smaller the denominator, and thus the larger the magnification.

To help you remember that you want a bigger focal length of the objective and a smaller one for the eyepiece to magnify your image, just look at the telescope itself and how it's designed! The eyepiece is tiny compared to the diameter of the telescope itself where the bigger objective sits.

picture of telescope

Reflecting Telescope

The eyepiece is important for the same reason in a reflecting telescope as in a refracting one. But the way the images are formed are just a bit different. Again, it helps to look at an image.

Diagram of Reflecting Telescope
diagram of reflecting telescope

In a reflector, the objective (a mirror as opposed to a lens) is located at the back of the telescope. Light hits the mirror and focuses at an imaginary point beyond a second mirror. This secondary mirror brings light to a focus from point 1 to point 2 on your screen. Just as before, the eyepiece takes over thereafter to help magnify the image for your eyes to appreciate.

Comparing Everything to the Human Eye

Your eyes can only appreciate so much of whatever they look at without the multiple powers of a telescope. During this lesson, I purposefully squeezed in a couple of important notes. Here was one of them:

  • The magnification of the image made by the objective is equal to the focal length of the objective lens divided by the focal length of the eyepiece lens.

When astronomers talk about magnification of a telescope, they are actually talking about the ratio of the size of an object as seen through a telescope compared to its size when seen with the naked eye.

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