How optical telescopes see the universe

For millennia, astronomers were limited by what they could see in the night sky – but this changed with the advent of telescopes. Jake Port explains how they work.

Perhaps the best-known optical telescope there is: the Hubble Space Telescope.

NASA’s Hubble Space Telescope has snapped many jaw-dropping images of the universe, one stands out: the iconic Pillars of Creation.

Taken in 1995 by the optical telescope, the image unveiled three mammoth columns of cold gas bathed in ultraviolet light blasting from young, massive stars in a region of the Eagle Nebula, some 7,000 light-years away.

How did Hubble manage to capture such an awe-inspiring image? It boils down to bending and bouncing light.

Telescopes can be broadly separated based on how they handle different wavelengths of light. This article will explain how optical telescopes, which use visible light to magnify and enhance an image, work. Those that use radio waves, infrared, ultraviolet, X-rays and gamma waves will be covered in their own explainer.

All optical telescopes focus light to a point that can be viewed with an eyepiece. Since there are three main classes of optical telescope, let’s look at each individually.

Refracting telescope

A large convex lens known as the primary or objective lens sits at the front of a hollow tube. The lens, which bulges in the middle, focuses parallel incoming light on a single point called the focal point.

The distance between the primary lens to the focal point is known as the focal length.

Just beyond the focal point in the eyepiece, a second, smaller convex lens – the secondary lens – reorients the light parallel again.

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A telescope’s power is mainly related to how much light it can take in. The bigger the objective lens, the brighter and clearer the image.

The secondary lens dictates its magnification, as it spreads the light focused by the objective lens over the viewer’s eye.

Reflecting telescope

The refracting telescope revolutionised astronomy. But as larger and larger telescopes were built, it became harder to produce a lens that was precise enough to produce a clear image.

This is because as a lens grows, imperfections in the production process become more pronounced, which blurs the image.

It also meant that the telescope had to be phenomenally long, as larger lenses needed longer focal distances.

This issue was solved in the 17th century by a number of physicists. In 1668, Sir Isaac Newton placed a large, curved mirror at the back of a tube and a smaller, diagonal mirror at the focal point which directed light towards an eyepiece on the side of the telescope.

Unlike refractors that focus light behind a primary lens, a reflector simply bounces the light towards the secondary mirror, which in turn directs the light at a 90-degree-angle towards the eyepiece.


With this setup, Newton could produce sharp images with a tube a fraction the length and size of a comparable refractor telescope.

These telescopes took off and are still popular today. Mirrors are far easier and cheaper to manufacture than lenses. And having the eyepiece on the side of the telescope also eliminates the need to stand – or, more often, crouch – at the end of the tube.

Catadioptric or compound telescope

The most common telescope used today combines the technologies of the reflector and refractor.

Known as a catadioptric telescope, it uses curved mirrors and lenses to focus light, but instead of directing the light to an eyepiece, the light travels through a small hole at the centre of the primary mirror.


This means an eyepiece, or a camera, placed at the end of this hole to capture the image. It’s the technique used by the Hubble Space Telescope, which has been responsible for capturing the most powerful visible-light images and changed the way we see the universe.

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