How do telescopes let us see so far into space?

Astronomer looking at the Milky Way

Everything you need to know about how telescopes work.

Why is your eye so bad at seeing things far away?

Human eyes can see long distances. In fact the Andromeda Galaxy can be seen with the naked eye and that's 2.5 million light-years away. But even a massive galaxy, like Andromeda, appears to us as a tiny point in the sky.

It makes sense that as an object gets further away it becomes harder to see. But why this happens helps us understand how vital telescopes have been in exploring the universe.

Diagram showing how the human eye works

As an object gets further away less of its light will reach your eye. The image takes up less space on your retina (the light-sensitive tissue at the back of your eye), making the image smaller. This makes details of the image harder to see.

Take a tour of the human eye and see how it works

Do bigger lenses give us a bigger image?

To make a distant object appear brighter and larger, we effectively need a bigger eye to collect more light. With more light we can create a brighter image, we can then magnify the image so that it takes up more space on our retina.

The big lens in the telescope (objective lens) collects much more light than your eye can from a distant object and focuses the light to a point (the focal point) inside the telescope.

A smaller lens (eyepiece lens) takes the bright light from the focal point and magnifies it so that it uses more of your retina.

Diagram showing how a reflecting telescope works

A telescope's ability to collect light depends on the size of the objective lens, which is used to gather and focus light from a narrow region of sky.

The eye piece magnifies the light collected by the objective lens, like a magnifying glass magnifies words on a page. But the performance of a telescope depends almost entirely on the size of the objective lens, sometimes called the aperture.

What's the big problem with refracting telescopes?

If you've ever seen light bend through a prism you probably have an idea of where the problem lies with a refracting telescope; it's the lens.


Take stunning space photos using a telescope

When light travels through glass it slows down, that's why it bends. Lenses are shaped perfectly to bend light in particular ways. But the amount light bends depends on the wavelength, or colour, of the light.

White light is a mixture of all colours, from red to violet. Red light bends the least and violet light bends the most.

When white light travels through the objective lens, the different colours bend at different angles and are focused at slightly different points. Different coloured images are misaligned creating a blurry image with fringes of colour along the boundaries that separate dark and bright parts.

Diagram showing abberation

Can telescopes with mirrors correct the problem?

Reflecting telescopes magnify distant objects using the same principle: more light is collected and focused to a point and this is magnified so that it fills your field of vision.

But instead of using a lens, a curved mirror (primary mirror) collects the light and reflects it to a focus. Because light doesn't pass through the mirror, it doesn't bend the different colours by different amounts, the way a refracting lens does.

A small mirror (secondary mirror) is placed in the path of light from the primary mirror to reflect the image towards the eyepiece. The secondary mirror must be very small so that it doesn't block the light from the distant object as it travels to the primary mirror.

Diagram showing how a refracting telescope works

Another benefit of using mirrors instead of lenses is that big mirrors are easier and cheaper to make than big lenses. Reflecting telescopes can be much larger and therefore look deeper into space.

Are radio telescope like big reflecting telescopes?

Jodrell Bank Observatory

Lovell telescope
  • Jodrell Bank Observatory was established in 1945 by Sir Bernard Lovell
  • Since then it's played an important role in researching meteors, quasars, pulsars, masers and gravitational lenses
  • Jodrell Bank's Lovell Telescope was used to track Sputnik 1, the first probe to go into space

Radio waves aren't just for listening to your favourite songs, they occur naturally all over the universe. In fact, they are a special type of light that humans can't see. They can be found emanating from clouds of gas where stars are born, as well as the centres of galaxies.

Many strong sources of radio waves are invisible in normal light, so looking at radio waves reveals a completely different picture of our universe. Even objects like the Sun and planets can reveal new features when viewed with radio telescopes, like Jodrell Bank.

Radio waves are also better at travelling long distances than shorter wavelengths, so we can get clearer signals from very distant objects in radio than we can in normal light.

The large dish acts like the primary mirror in a reflecting telescope, but it needs to be much larger to reflect the long wavelength radio waves. These are reflected up to a smaller mirror which reflects the images back to a receiver. The information from the receiver is then processed by computers to create colour images which we can see.

Diagram showing how radio telescopes work

Why bother putting a telescope in space?

The problem with telescopes on Earth is that light being received has to pass through our atmosphere. Although a clear sky may seem transparent, light can be scattered by the molecules in our air. This is what makes stars twinkle!

The atmosphere also blocks certain kinds of light, including X-rays, ultraviolet and infrared light. This effect is beneficial for life on Earth, but to see these kinds of light from stars and galaxies, we need to venture outside of our atmosphere.

That's why the Hubble telescope can take such incredible images of distant galaxies, allowing us to explore the history of our universe.

Watch Brian Cox uses Hubble to see galaxies 13 billion years old


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