Why No One Has Built a Bigger Telescope Lens Since 1897

By James · September 7, 2026

I look forward to fall nights before summer even ends. Once the crickets start up in the evening, I know the season is turning, and some part of me starts waiting for the sky to clear.

The reason goes back to my father. About forty years ago, he spent what was serious money at the time on a telescope. The first time I looked through that lens, at the moon's edge and a smear of starlight and more sky than I knew existed, the curiosity probably started right there.

That telescope was a refractor. It gathered light through a glass lens, the same basic idea Galileo used four hundred years earlier. What I didn't know then is that lenses had already lost this argument at the top end of astronomy, and had lost it a lifetime before he handed me that one. That still strikes me as strange. Glass and grinding have only gotten better since.

A father and young son observing the Moon and stars through a vintage astronomical telescope on an autumn night

The biggest telescope lens ever put to serious use is the 40-inch at Yerkes Observatory in Wisconsin, finished in 1897. Nobody has beaten it, and the reason is structural rather than financial. Light has to travel through a lens, so a lens can only be held at its rim, and past a certain size it sags under its own weight. A mirror can be braced across its entire back. That single difference is why nearly every major telescope built since, on the ground and in orbit, gathers its light with a mirror instead.

Why the Largest Refracting Telescope Ever Built Dates to 1897

That lens my father bought, the one that first showed me the moon, belongs to a family of instruments that hit a wall in 1897 and never got past it.

That family was no sideline. It was the mainstream. A survey of the great nineteenth-century refractors in Experimental Astronomy traces the climb from Fraunhofer's 23-centimeter objective in 1824 and finds it close to a straight climb, decade after decade, with every serious observatory built around a lens. Then it stopped.

It stopped in Williams Bay, Wisconsin. The Yerkes 40-inch lens, just over a meter across, was ground from crown and flint glass shipped in from Paris and weighs about 500 pounds. For the eleven years after it was finished, no telescope of any kind on Earth was larger.

Yet the size that set the record is also what doomed the design, and the trouble came from two directions at once. Glass refracts different wavelengths by slightly different amounts, which can smear a star's point of light into a faint rainbow fringe. Astronomers call it chromatic aberration. It appears in a lens of any size; what changes with scale is how hard it becomes to correct.

The second problem is plainer. Light has to pass through the full thickness of a lens, so the glass can only be held near its rim, the way eyeglass frames grip a lens at the edge. OpenStax's college astronomy textbook spells out what follows: unsupported in the middle, a big lens droops just enough to bend the very light path it exists to keep sharp.

I spend my working life around metal and measurement, and this is the half that lands hardest for me. Sag like that does not scale politely. Double the span and the droop grows by several times, so every extra inch of aperture costs more than the inch before it. You cannot inspect your way out of it either, because the amount of sag changes as the instrument tips toward a different part of the sky.

The 1900 Paris Refractor: A Bigger Lens That Could Never Be Aimed

There is one exception, and it makes the case better than the record does.

For the Paris Universal Exhibition of 1900, French engineers built a refractor with a 1.25-meter objective, comfortably larger than the Yerkes lens. They could only build it at all by giving up on pointing it. Its focal length ran to 57 meters, the tube was mounted horizontally and could not move, and starlight had to be fed into it by a steerable flat mirror two meters across, a siderostat, standing at the front like a hand cupped to an ear.

It drew crowds, which was the point, but no astronomer had any use for it. Nobody wanted to buy it afterward either, and most of the thing was sold off as scrap metal. The two objective lenses survive, boxed away in the Paris Observatory.

That same survey treats Yerkes and the Paris giant as the last two of the breed. They were finished within three years of each other, and no larger astronomical objective lens was built after them. The difference between the pair is simply that one of them could still be aimed at a star.

By then the argument had moved on. Mirrors had become the less expensive way to gather light as well as the better one, and the 40-inch turned out to be the last name on the list. The Vatican Observatory's account of the instrument makes the point without ceremony: nothing bigger ever entered service.

So the largest working lens in astronomy's history sits in Wisconsin, unbeaten for well over a century, not because nobody wanted a bigger one, but because bigger, for a lens, meant worse. The fix had been sitting in plain view for more than two centuries, in a tube you could hold in one hand.

A comparison of a large refracting telescope lens and a giant reflecting telescope mirror supported by precision actuators

How Newton's 1.3-Inch Mirror Solved What a 40-Inch Lens Could Not

That fix belonged to Isaac Newton, and it was almost comically small.

He built it in 1668, and the primary mirror measured about 1.3 inches across, roughly the diameter of a half-dollar coin. Newton cast it himself from speculum metal, an alloy of copper and tin with a trace of arsenic he added because he believed it gave the surface a whiter shine, then ground and polished it by hand. The Science Museum's account of the craft quotes his own warning about not breathing the fumes while the stuff melted.

That sliver of metal still magnified about 40 times, and Newton reported seeing Jupiter's four largest moons and the crescent shape of Venus through it. Unlike a single-element lens of the same size, it introduced no chromatic aberration of its own, because reflection does not spread wavelengths apart the way refraction through glass does.

The design was not perfect out of the gate. Early speculum-metal mirrors tarnished quickly and lost far more of the incoming light than a good lens passed through, so for a long stretch afterward, big refractors kept real advantages.

The same trade-off, in miniature, is still the first decision anyone makes when buying their first telescope: a cheap refractor's color fringing, or a reflector's slightly trickier upkeep.

What settled the argument in the long run was structural. A lens has to be right all the way through: every surface the light crosses, and every millimeter of glass in between, free of streaks and bubbles. A mirror only has to be right on one face. Everything behind that face is just support, so it can be ribbed, hollowed out, or propped from a hundred points at once, whatever keeps the front surface honest. That is an enormous amount of freedom.

A mirror still has to be cast, cooled, and polished as a single disk of glass, though, and there is a size where even that stops being possible.

Engineers standing beside a massive segmented primary mirror made of hundreds of precision mirror segments

Why Big Telescope Mirrors Are Built From Hundreds of Segments

Jerry Nelson ran headlong into that limit in the 1980s, and his answer was to stop pouring one enormous mirror at all.

Working out of the Lawrence Berkeley Laboratory with Terry Mast, he designed the primary mirror for what became the W. M. Keck Observatory on Mauna Kea as 36 hexagonal segments rather than one giant disk, each about 1.8 meters across. Plenty of astronomers thought it could not work. Caltech's oral history of the project records that the sharpest doubts in the field landed on exactly this claim: that separate segments could be held in step well enough to behave like one unbroken surface.

They can. Sensors and computer-controlled actuators hold each segment relative to its neighbors to within about four nanometers, correcting as the telescope tips and tracks, by the observatory's own account.

Four nanometers is a number I have trouble taking seriously. In a good shop, the micrometer is the unit you argue about. Keck is holding its segments roughly 250 times tighter than that, outdoors, on a mountaintop, while the whole structure swings around to track a star.

The trick works because of how a mirror handles light in the first place. Reflection happens entirely at the front surface, so once every segment is shaped correctly and phased with its neighbors to a fraction of a wavelength, the assembly performs, for scientific purposes, like a single unbroken mirror.

A lens makes a much harder candidate. Light travels all the way through a lens instead of bouncing off the front. A Keck-scale segmented lens would need every piece matched in curvature and in internal glass quality, on every face, with the seams controlled well enough not to scatter or bend light unevenly. Smaller multi-element and segmented lens systems exist elsewhere in optics, but nobody has built a precision segmented lens at anything like Keck's scale. Segmenting a mirror divides a surface. Segmenting a lens would divide a volume.

The precision involved is hard to picture. Keck offers one way in: blow a single segment up to the size of the planet, and the roughest spots left on its surface would still rise only about three feet.

The ceiling Nelson broke was a casting problem, not an optical one. Mirrors had been growing steadily for two centuries before him: Herschel's 49.5-inch reflector in 1789, Lord Rosse's 72-inch Leviathan in 1845, the 100-inch Hooker telescope in 1917.

They topped out at Palomar's 200-inch Hale Telescope in 1948, the largest monolithic mirror anyone had yet built. It stayed the world's largest fully functional telescope for forty-five years, until Keck's segmented design passed it in 1993.

Ten meters was suddenly achievable. The bigger prize sat above the atmosphere entirely, an argument Nancy Grace Roman had been pressing since the 1960s, and in orbit the same segmenting idea could go much further.

Starlight reflected by a large primary mirror with active support actuators and sensors maintaining its precise shape

From Newton's 1.3-Inch Mirror to the 39-Meter Extremely Large Telescope

The telescope that idea produced is in orbit right now, unfolded from a rocket fairing.

The James Webb Space Telescope's primary mirror spans 6.5 meters and consists of 18 hexagonal beryllium segments, each about 1.32 meters across under a gold coating roughly 100 nanometers thick, according to NASA. Six actuators sit behind every segment. Engineers aligned all 18 to a small fraction of the width of a human hair, so the set would work as one.

Back on the ground, the European Southern Observatory's Extremely Large Telescope pushes the idea to its current limit. Under construction on Cerro Armazones in Chile's Atacama Desert, its 39-meter primary mirror will be built from 798 hexagonal segments, each about 1.4 meters across and only 5 centimeters thick.

Not every modern telescope goes the segmented route. The Vera Rubin Observatory ground its 8.4-meter primary and 5-meter tertiary mirrors from a single piece of glass. A lightweight honeycomb backing keeps the disk from being too heavy to move or too slow to cool evenly, as the observatory's own account of the build explains. It ranks among the largest single-substrate mirrors ever made, at an observatory named for the astronomer whose work on galaxy rotation first pointed to dark matter.

Line the two extremes up and the scale gets hard to hold in your head. The lens at Yerkes measures 1.02 meters across. The Extremely Large Telescope's mirror will measure 39 meters, which by aperture area alone gathers roughly 1,450 times as much light from the same patch of sky.

Every one of those 798 segments will do what Newton's 1.3-inch mirror did in 1668: reflect light without splitting its colors, braced from behind rather than pinched at the edge. The physics never changed. What changed is how many pieces astronomers were willing to break the mirror into to get there.

A scale comparison of a 1.02-meter telescope lens, the 6.5-meter James Webb Space Telescope mirror, and a 39-meter-class primary mirror

Attribute	Refracting Telescope (Lens)	Reflecting Telescope (Mirror)
Chromatic aberration	Inherent unless corrected with specialized multi-element optics	Absent from the reflection itself
How it's supported	Mainly near the edge, to keep the aperture clear	Can be braced across the full back surface
Surfaces that must be perfect	Every surface the light crosses, plus the glass in between	One front face; everything behind it is structure
Segmented large-aperture designs	Impractical at precision-astronomy scale	Proven at large observatories, actively aligned
Largest example put to work	Yerkes Observatory, 1.02 m (1897), still unbeaten	Extremely Large Telescope, 39 m (under construction)

I think about Johannes Hevelius sometimes. He served as mayor of Danzig and, in whatever hours the job left him, chased a sharper view of the sky. His largest instrument was a 140-foot tubeless telescope, a lens carried on a mast at an observing post outside the city, aimed and steadied with ropes and pulleys. He drew up plans for a 150-foot version, needing a tower over a hundred feet tall to handle it, and never built it.

The part that stays with me came out of the archives much later. The scholarship behind the UNESCO and IAU heritage record of his observatory concludes that the 140-foot telescope was probably never used for observations at all. He had the tube made before the lens existed. When the finished lens came back to him, it needed a longer tube than the one waiting for it. The largest telescope of its age may never have shown anyone a thing.

Seen today, the rig looks absurd, a contraption swaying in the Baltic wind. Newton had already ground his little mirror by the time Hevelius put the giant into print, and the mirror is the one that won. But the appetite behind both is what runs through this whole story: from a palm-sized disk of speculum metal, to Palomar's 200 inches, to the 798 pieces being assembled in the Chilean desert. It's also the appetite that put a telescope in my father's hands.

The material kept changing. Glass, then metal, then glass again cut into hundreds of pieces. The reason for building never did. I can still feel that autumn evening, tilting my father's telescope toward the sky, waiting for my eyes to adjust to whatever was on the other side of the lens. Hevelius, it seems, never stopped waiting either.

Johannes Hevelius beside a historic long-focus telescope contrasted with a modern extremely large telescope under a star-filled night sky

Frequently asked questions

Do any modern telescopes still use lenses?

Yes, mainly smaller ones. Refracting telescopes remain common in amateur astronomy and as finder scopes, where apertures stay under roughly six inches, and both sag and chromatic aberration stay manageable at that size.

Why not just launch a giant lens telescope into space, where gravity isn't a problem?

Weightlessness removes the sag problem, but not the color problem, and a lens still needs a flawless piece of glass ground to a precise curve on every surface. The large space telescopes built for deep imaging, Hubble and the James Webb Space Telescope among them, use a mirror for exactly that reason.

How much bigger can telescope mirrors get?

Segmenting removes the need to cast one enormous piece of glass, but real limits remain: aligning hundreds of segments, thermal and wind-driven flexing, active-optics systems, dome size, and budget. The Extremely Large Telescope's 39-meter mirror is currently the largest under construction.

Sources & References

This article is for educational and informational purposes only. Sources are linked where available. Readers are encouraged to consult primary sources for further research.

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