Far-Seeing - How Do Telescopes Work?

A simple refracting telescope peers into the distant universe.

Ask any number of people what the single most important tool in astronomy is, and almost universally, they will claim the telescope as their answer. With telescopes, we’ve been able to peer beyond our world and out into the distant reaches of the universe. The name literally means far-seeing. How is it that these rather simple contraptions are able to show us so much? How do telescopes work?

The First Telescopes

The earliest record of a telescope comes from 1608. Believe it or not, it was not Galileo who invented it. The inventor of the telescope is unknown. At first, telescopes weren’t used for peering into the cosmos, but instead for far more earthly purposes, such as navigation. But when the famous astronomer Galileo Galilei heard of the device in 1609, he realized its true potential, built his own, and pointed it at the sky.

Galileo’s telescope was simple by modern standards. It was what we now call a refracting telescope. Such telescopes are typically two distinct lenses connected together by a long tube. The front of the telescope holds a large lens called the objective lens. Its job is to gather light from a distant object and bend, or refract, that light to a single point of focus. At the other end of the tube is a smaller lens called the eyepiece. It works like a magnifying glass and spreads the incoming light outwards to make the object in view appear larger, and therefore closer, while also focusing it so the eye receives a clear image.

A simple diagram showing how a refracting telescope works. Image Credit: NASA/JPL-Caltech

The objective and the eyepiece work together to create a focused, enlarged image. The objective gathers more light from the target object, allowing for a clearer image, while the eyepiece is what enlarges the image. One without the other would not create a clear image. The tube is required because the two lenses need to be kept at a specific distance apart from each other and it maintains their precise alignment. Any variation in these would lead to a blurry view.

Despite Galileo’s discoveries with his refracting telescope, there were many shortcomings to the design. The biggest issue was the limit on size. The further you want to see with a refracting telescope, the bigger a lens you would need. The strength of a telescope comes not from its magnification power, but from how much light you can gather. More light doesn’t just mean a brighter image, it also means more detail captured. An extremely zoomed-in image is nothing if it’s incredibly dim and lacking detail. Telescopes operate like light buckets. They gather light just like regular buckets gather rain. The wider a bucket, the more raindrops it will capture, and the more water that will be collected. For a telescope, the wider its lens’ width, known as its aperture, the more light it will gather.

The problem with refracting telescopes is that large lenses are not only significantly more bulky and difficult to work with, but they’re also far harder to make and therefore more expensive. To truly gaze out in the universe, a different approach was needed.

An Improved Design

It was Sir Isaac Newton who discovered the solution to this problem in 1668 with his invention of the reflecting telescope. Newton cut out the objective lens altogether and instead used a curved mirror to gather light before passing it to the eyepiece. To further the earlier analogy, a reflecting telescope even looks like a bucket. A large open tube has the primary mirror at the back. The mirror does what mirrors do and reflects all the light hitting it forward, but due to its concave shape, it focuses the light into a smaller area, just like the refracting objective lens did. This light then bounces off another mirror, a flat one known as the secondary mirror, and is redirected into the eyepiece, which works the same as the refracting telescope eyepiece.

A simple diagram showing how a reflecting telescope works. Image Credit: NASA/JPL-Caltech

The reflecting telescope is far easier to manufacture and the mirror can be made much larger than a lens. It also has advantages in size. As the path the light follows bouncing off of the two mirrors can be longer than the tube itself, the tube of a reflecting telescope can be shorter than the length of an equally powerful refracting telescope’s tube. The compact nature of the reflecting telescope makes it far easier to scale up to larger sizes as well.

Given these advantages, most telescopes used to this day, from an amateur astronomer’s to an observatory’s, are reflecting telescopes, rather than refracting. Space telescopes, like the Hubble or James Webb Space Telescope, are just massively scaled up reflecting telescopes. Even radio telescopes, which look like giant satellite dishes, are technically just another type of reflecting telescope.

The telescope, even in its modern incarnation, is a deceptively simple piece of technology, and yet it has helped to put the secrets of the universe within our sight. It could be argued that no other scientific instrument has shown us as much as the humble telescope, all without significantly changing in design for nearly 400 years.