How Does a Telescope Work?

Milky-Way.Kiwi
Apr 28 · 6 min read
A simple refracting telescope looking at the Moon

Telescopes are simple, they are a tube with some glass. There are a lot more details to them, but fundamentally they are a tube with either a lens or a mirror at one end and an eyepiece at the other end. The basic design has not changed very much since Galileo rolled out his first telescope in 1609 and Newton displayed his reflecting telescope in 1672.

The job of the telescope is to take the light from a patch of sky and cram it into your eye.

Initially this was so we could see, with our own eyes, the amazing sights of the night sky that were not visible to the naked eye. Now we attach a huge range of complex instruments to telescopes to capture data that helps uncover how the universe works. Over the next few words, and pictures, we will cover the basics of how telescopes work.

The good new is there is no mathematics or equations to worry about.

The Parts

Before we start it is worth familiarising ourselves with the basic parts of a telescope. We will deal mostly with the type of telescope called a refractor. This is a telescope with a large lens at the front of a tube and small eyepiece at the other end that an observer looks through.

The main parts of a refracting telescope.

Parallel Light

The best place to start, when talking about telescopes, is the night sky. This is because that is why we have a telescope — because we want to look at the night sky.

The first concept to understand is that light from night sky objects hits us as parallel rays of light.

This is because objects, like stars, are so far away that there is no discernible divergence or convergence in the light that makes it to the Earth. Our eyes like this as we can focus parallel light and see nice clear crispy point like stars.

The red tubes going into the telescope represent light rays — they are parallel going into the telescope and they are parallel going out of the telescope.

A telescope has to take parallel light rays, bend them through either reflection or refraction (as in this telescope) and then deliver parallel light rays to our eyes. That’s a key point to remember, you start with parallel light rays and you have to finish with parallel light rays.

What happens inside

What makes the amazing objects of the night sky come to life is because of whats happens inside the telescope. For a refracting telescope it all begins when the light hits the outside of the objective lens (the big bit of glass at the front). Let’s look under the hood:

Here’s our refractor with the tube chopped in half to show what is happening inside.

The light that hits the objective lens is refracted, bent towards the middle. This means that the light after the objective lens is no longer parallel. The light rays converge on each other at a point that is not far from the eyepiece, just in front of it. Where this happens is called the focal plane.

After the focal plane the light rays diverge again until they hit the eyepiece, whose job it is to make the light rays parallel for your eye. The light that finally leaves the telescope is parallel again.

Here’s the top view with the red shaded areas showing the path of parallel light rays that are hitting the objective lens straight on.

The Tricky Bit

The next bit gets a bit tricky. In the above picture the light rays are perpendicular to the objective, that means they are hitting the objective straight on and parallel. For this to occur the light rays would have to be coming from something that was in the middle of the view that the telescope was looking at.

If this was a star in the middle of the field of view, it is so far away that even at the edge of the objective lens the light rays from that star would still be hitting the objective parallel and straight on. At every point on the objective lens the light rays from that star are hitting the lens parallel and straight on.

If you looked into our half telescope and looked towards the objective it might look like this, with a whole lot of stars and one conveniently in the middle:

This is how the view might look if we were in the telescope tube and looked towards the objective lens.

For a star that is towards the left edge of the field of view (the shaded circle in the above picture representing the view through the objective above) the light rays from that star will hit the objective parallel, but at an angle. The path of the light would look something like this:

The light rays from a star to the left of the field of view would still hit the objective parallel, but they would all hit at an angle.

In the above picture the light rays are converged after going through the objective lens to a point on the focal plane that is to the right of the centre. For a star to the right of the field of view the same happens but on the other side, the parallel light rays would strike the objective at an angle from the right. The objective would then converge the light rays to a point that is to the left of the centre of the focal plane.

The light rays from a star to the right of the field of view will converge to a point that is left of the centre of the focal plane (the focal plane is where the light rays converge to a point, just inside the telescope from the eyepiece).

What this also shows is how the objective lens of the telescope rotates the view by 180 degrees. A point on the left in the sky will become a point on the right at the focal plane, a point on the right in the sky will become a point on the left on the focal plane. Every single point that is in the field of view will act in this way, each of them will converge to their own corresponding point on the opposite side of the focal plane.

If the light from all of the three point could be seen, it might look like the this.

The Final Piece

The final piece that gets the light back out of the telescope and into your eye is the eyepiece. Its job is to allow you to see all of those points on the focal plane. Depending on the eyepiece they also allow you examine the information on the focal plane by zooming in. All that is happening with the eyepiece is that it is letting you see more or less of the light that is passing through the focal plane.

Where the three light ray cones converge is the site of the focal plane. The job of the eyepiece is to turn those diverging rays of light back into streams of parallel light.

The End

It’s not quite the end of the story, there is a lot more to it, but for now this is enough.

Milky-Way.Kiwi

Written by

Milky-Way.Kiwi is Haritina Mogoșanu and Samuel Leske. We are two New Zealanders who love writing and talking about space.

Why bother with Space

Why venturing beyond Earth should be on our priority list

Milky-Way.Kiwi

Written by

Milky-Way.Kiwi is Haritina Mogoșanu and Samuel Leske. We are two New Zealanders who love writing and talking about space.

Why bother with Space

Why venturing beyond Earth should be on our priority list

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