How can the James Webb Space Telescope see so far?

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How does the camera on the James Webb Space Telescope work and see so far out? – Kieran G., age 12, Minnesota
Imagine a camera so powerful it can see light from galaxies that formed more than 13 billion years ago. That's exactly what NASA's James Webb Space Telescope is built to do.
Since it launched in December 2021, Webb has been orbiting more than a million miles from Earth, capturing breathtaking images of deep space. But how does it actually work? And how can it see so far? The secret lies in its powerful cameras – especially ones that don't see light the way our eyes do.
I'm an astrophysicist who studies galaxies and supermassive black holes, and the Webb telescope is an incredible tool for observing some of the earliest galaxies and black holes in the universe.
When Webb takes a picture of a distant galaxy, astronomers like me are actually seeing what that galaxy looked like billions of years ago. The light from that galaxy has been traveling across space for the billions of years it takes to reach the telescope's mirror. It's like having a time machine that takes snapshots of the early universe.
By using a giant mirror to collect ancient light, Webb has been discovering new secrets about the universe.
Unlike regular cameras or even the Hubble Space Telescope, which take images of visible light, Webb is designed to see a kind of light that's invisible to your eyes: infrared light. Infrared light has longer wavelengths than visible light, which is why our eyes can't detect it. But with the right instruments, Webb can capture infrared light to study some of the earliest and most distant objects in the universe.
Although the human eye cannot see it, people can detect infrared light as a form of heat using specialized technology, such as infrared cameras or thermal sensors. For example, night-vision goggles use infrared light to detect warm objects in the dark. Webb uses the same idea to study stars, galaxies and planets.
Why infrared? When visible light from faraway galaxies travels across the universe, it stretches out. This is because the universe is expanding. That stretching turns visible light into infrared light. So, the most distant galaxies in space don't shine in visible light anymore – they glow in faint infrared. That's the light Webb is built to detect.
Before the light reaches the cameras, it first has to be collected by the Webb telescope's enormous golden mirror. This mirror is over 21 feet (6.5 meters) wide and made of 18 smaller mirror pieces that fit together like a honeycomb. It's coated in a thin layer of real gold – not just to look fancy, but because gold reflects infrared light extremely well.
The mirror gathers light from deep space and reflects it into the telescope's instruments. The bigger the mirror, the more light it can collect – and the farther it can see. Webb's mirror is the largest ever launched into space.
The most important 'eyes' of the telescope are two science instruments that act like cameras: NIRCam and MIRI.
NIRCam stands for near-infrared camera. It's the primary camera on Webb and takes stunning images of galaxies and stars. It also has a coronagraph – a device that blocks out starlight so it can photograph very faint objects near bright sources, such as planets orbiting bright stars.
NIRCam works by imaging near-infrared light, the type closest to what human eyes can almost see, and splitting it into different wavelengths. This helps scientists learn not just what something looks like but what it's made of. Different materials in space absorb and emit infrared light at specific wavelengths, creating a kind of unique chemical fingerprint. By studying these fingerprints, scientists can uncover the properties of distant stars and galaxies.
MIRI, or the mid-infrared instrument, detects longer infrared wavelengths, which are especially useful for spotting cooler and dustier objects, such as stars that are still forming inside clouds of gas. MIRI can even help find clues about the types of molecules in the atmospheres of planets that might support life.
Both cameras are far more sensitive than the standard cameras used on Earth. NIRCam and MIRI can detect the tiniest amounts of heat from billions of light-years away. If you had Webb's NIRCam as your eyes, you could see the heat from a bumblebee on the Moon. That's how sensitive it is.
Because Webb is trying to detect faint heat from faraway objects, it needs to keep itself as cold as possible. That's why it carries a giant sun shield about the size of a tennis court. This five-layer sun shield blocks heat from the Sun, Earth and even the Moon, helping Webb stay incredibly cold: around -370 degrees F (-223 degrees C).
MIRI needs to be even colder. It has its own special refrigerator, called a cryocooler, to keep it chilled to nearly -447 degrees F (-266 degrees C). If Webb were even a little warm, its own heat would drown out the distant signals it's trying to detect.
Once light reaches the Webb telescope's cameras, it hits sensors called detectors. These detectors don't capture regular photos like a phone camera. Instead, they convert the incoming infrared light into digital data. That data is then sent back to Earth, where scientists process it into full-color images.
The colors we see in Webb's pictures aren't what the camera 'sees' directly. Because infrared light is invisible, scientists assign colors to different wavelengths to help us understand what's in the image. These processed images help show the structure, age and composition of galaxies, stars and more.
By using a giant mirror to collect invisible infrared light and sending it to super-cold cameras, Webb lets us see galaxies that formed just after the universe began.
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This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Adi Foord, University of Maryland, Baltimore County
Read more:
Could a telescope ever see the beginning of time? An astronomer explains
How the James Webb Space Telescope has revealed a surprisingly bright, complex and element-filled early universe – podcast
James Webb Space Telescope: An astronomer explains the stunning, newly released first images
Adi Foord does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.