Comparison of space telescope mirror sizes

It All Comes Down to a Big Piece of Glass

Let’s be honest. When we see a new, stunning image from a space telescope, we don’t usually think about its mirror first. We’re captivated by the swirling galaxies, the birth of stars, the haunting beauty of a nebula. But every single one of those photons, every scrap of light that traveled millions of years to create that picture, had to hit one thing first: the telescope’s primary mirror.

Think of it as the telescope’s lightbucket. The bigger the bucket, the more rain you catch. In this case, the “rain” is starlight. A larger mirror doesn’t just make things brighter; it allows us to see fainter, more distant objects and resolve finer details. It’s the difference between squinting at a streetlamp through a foggy window and having a frontrow seat at the cosmic opera.

I remember the first time I saw the Hubble Space Telescope’s mirror in person at the Smithsonian. It’s a backup, but it’s massive. You stand there, looking at this perfectly polished, 2.4meter (about 8foot) disk of glass and coatings, and it hits you. This is the thing that rewrote our astronomy textbooks. This single piece of engineering showed us the age of the universe and revealed galaxies at the very edge of observable time. All because of its size and precision.

Why Size is the Name of the Game in Space

On Earth, amateur astronomers are always dreaming of a bigger scope. I’ve got a friend, Dave, who started with a 4inch reflector in his suburban backyard. He was thrilled to see the rings of Saturn. Then he upgraded to a 10inch. Suddenly, he wasn’t just seeing Saturn’s rings; he was spotting the Cassini Division—the dark gap between them. He could make out cloud bands on Jupiter. That’s the power of aperture.

Now, imagine that principle, but without our blurry, turbulent atmosphere getting in the way. In space, a telescope’s resolution is limited almost entirely by the size of its mirror and the quality of its optics. The math is pretty straightforward: the larger the mirror’s diameter, the finer the detail it can see and the fainter the objects it can detect.

It’s not just about bragging rights, though. A bigger mirror is a time machine. When we point a massive telescope at a faint smudge of light, we’re collecting photons that have been traveling for billions of years. We’re literally looking back in time. The James Webb Space Telescope’s colossal mirror was specifically designed to catch the incredibly faint infrared light from the very first stars and galaxies that ever formed. Without its immense size, that mission would be impossible.

The Big Players: A Lineup of Cosmic LightBuckets

Let’s meet the giants. We’ll start with the legend and work our way up to the new king on the block.

Hubble Space Telescope: The People’s Champion

Mirror Size: 2.4 meters (7.9 feet)

Hubble is the telescope that brought the cosmos into our living rooms. Its 2.4meter mirror is, by modern standards, not exceptionally large. Many groundbased observatories dwarf it. But what Hubble lacked in raw size, it made up for with its pristine location in orbit. For decades, it was our crystalclear window to the universe.

That 2.4meter size wasn’t arbitrary. It was the largest mirror they could build that would still fit inside the Space Shuttle’s cargo bay. A perfect example of engineering constraints shaping science. And despite its “modest” aperture, its impact is immeasurable. It proved that if you want a perfectly stable, atmospherefree view, you have to go to space.

James Webb Space Telescope: The New Gold Standard

Mirror Size: 6.5 meters (21.3 feet)

Here’s where things get crazy. Webb’s mirror isn’t just bigger than Hubble’s; it’s a completely different beast. To fit inside a rocket fairing, it had to be foldable. Webb’s primary mirror is composed of 18 hexagonal segments made of beryllium, each covered in a thin layer of gold optimized for reflecting infrared light.

When fully deployed, it has a total collecting area of about 25 square meters, compared to Hubble’s 4.5. That’s over six times the lightgathering power. Let that sink in. It can see objects nearly 100 times fainter than what Hubble can detect. This is why its first deep field image was so mindblowing—it wasn’t just a pretty picture; it was a datarich snapshot of galaxies so distant, their light is stretched into the infrared by the expansion of the universe.

Nancy Grace Roman Space Telescope: The WideAngle Specialist

Mirror Size: 2.4 meters (7.9 feet)

You might look at that and think, “Hey, that’s the same size as Hubble’s.” And you’d be right. But the Roman Space Telescope, named after NASA’s first chief astronomer, is built for a completely different purpose. While Webb is a powerful but narrowfocused infrared observatory, Roman is a survey telescope with a field of view 100 times larger than Hubble’s.

Think of it this way: If Hubble is a telephoto lens perfect for zooming in on a single bird, Roman is a wideangle lens that can capture the entire flock. Its mirror size is a sweet spot—large enough to gather plenty of light from distant galaxies, but part of an optical system designed to map huge swaths of the sky with incredible efficiency. It’s going to revolutionize cosmology by taking a massive cosmic census.

The TradeOffs: It’s Not Just About Going Bigger

So, why don’t we just build a 20meter mirror and be done with it? Well, it’s complicated. And expensive.

Every decision is a balancing act. A larger mirror is heavier, which dramatically increases launch costs. It’s more complex to build, polish, and coat to the nanometerlevel precision required. And then there’s the problem of getting it into space. Webb’s segmented, folding design was a monumental engineering challenge that had never been attempted before on this scale.

There’s also the question of mission. A telescope designed to study the chemistry of exoplanet atmospheres, like Webb, needs a huge mirror to collect enough light from a single, tiny, distant world. A telescope designed to map dark matter, like Roman, needs a different kind of optimization—a wider field of view is more critical than sheer lightgathering power for a single object.

You can get a deeper dive into the engineering marvel of Webb’s mirror on NASA’s official Webb telescope mirrors page.

What’s Next? The Future is Even Bigger

The march toward larger mirrors isn’t stopping. Astronomers are already dreaming of the next generation. Concepts for the LUVOIR (Large UV/Optical/IR Surveyor) mission envision a telescope with a mirror as large as 15 meters—nearly as big as the entire James Webb spacecraft is wide.

The goal? To directly image Earthlike exoplanets and search for signs of life. To do that, you need a mirror so large and precise that it can block out the blinding light of a parent star and see the faint, pale blue dot orbiting it. That’s the holy grail. And it will require a mirror on a scale we can barely conceive of building today.

For a look at how groundbased telescopes are also pushing the limits of size (which informs space telescope design), check out the Giant Magellan Telescope project, which will use seven of the world’s largest mirrors acting in concert.

Your Questions, Answered

Why can’t we just use one giant, solid mirror?

It mostly comes down to rockets. Rocket fairings—the nose cones that carry the payload—are only so wide. A giant, solid mirror that’s, say, 10 meters across simply wouldn’t fit. That’s why segmented and folding mirrors, like Webb’s, are the future. They’re complex, but they’re the only way to get a massive lightcollecting surface into space.

Is a bigger mirror always a better telescope?

Not necessarily. It depends on the science. A bigger mirror is better for seeing faint, distant point sources, like the first galaxies. But for widefield surveys that map large areas quickly, a different optical design with a slightly smaller mirror might be more efficient and costeffective. It’s about matching the tool to the job.

What’s the largest mirror ever launched?

As of now, that title belongs to the James Webb Space Telescope and its 6.5meter primary mirror. It smashed the previous record held by Hubble. The next recordholder will likely be one of the large concept missions currently being studied by NASA and other space agencies.

How do they make mirrors so precise?

It’s a painstaking process. They start with a highly stable material like beryllium or a special glassceramic. Then, they grind and polish it for months or even years, using incredibly accurate lasers to measure the surface. The final polish has to be perfect to within a fraction of the wavelength of light. If Webb’s mirror were scaled up to the size of the United States, the biggest imperfection would be only a few inches high.

So, the next time you lose yourself in a jawdropping image from space, take a second to think about the incredible piece of engineering that made it possible. That beautiful picture started its journey as a few precious photons, falling into a giant, silent bucket, millions of miles from home. And as our buckets get bigger, our universe just gets more astonishing.