What is the largest refracting telescope?

The King of Glass: Exploring the World’s Largest Refracting Telescope

If you’re anything like me, the idea of massive telescopes just gets you. And when it comes to sheer size and historical significance, one telescope stands head and shoulders above the rest: the 40-inch refractor at Yerkes Observatory in Wisconsin. Yep, that’s right – Wisconsin! This behemoth, finished way back in 1897, still holds the crown as the largest refracting telescope ever put to serious astronomical use. Pretty cool, huh?

A Colossus of Glass and Steel

Now, refracting telescopes use lenses to gather light, a concept that’s been around since the early 1600s. Think about that for a second – people were grinding lenses and peering at the heavens centuries ago! While some Dutch spectacle maker, Hans Lippershey, was the first to try and patent one, it was folks like Galileo who really ran with the idea, turning these early scopes towards the stars and changing astronomy forever. Over time, as lens-making got better and telescope designs evolved, instruments just kept getting bigger and bigger.

And that’s where Yerkes comes in. This telescope is basically the ultimate expression of that era. The whole thing was the brainchild of astronomer George Ellery Hale, who had this grand vision of a cutting-edge research facility. He managed to rope in a wealthy businessman named Charles T. Yerkes to foot the bill, and together, they set out to build something truly special. At the heart of it all was this massive refracting telescope, one that would make all the others look like toys.

How Do You Build a Giant?

Building the Yerkes telescope was no walk in the park. It was a real feat of engineering and precision. The glass for the lenses was initially cast by Mantois in Paris, France, and then shipped across the Atlantic to Alvan Clark & Sons in Massachusetts. Can you imagine the logistics of that back then? The Clarks then painstakingly ground and polished the glass into what would be the largest telescope lens ever made. That lens is an achromatic doublet, which basically means it’s made of two pieces of glass to help correct for color distortion. The whole thing weighs around 225 kilograms!

This beast boasts a 40-inch (102 cm) aperture and a focal length of 19.3 meters, which gives it a focal ratio of f/19. The lenses had to be flawless, absolutely perfect, to work correctly. Clark had to use clay rods to continuously stir the molten glass in the furnace, preventing bubbles and ensuring the lenses were free of defects. A 62-foot-long steel tube supports the lenses at the front and the eyepiece at the back.

But it doesn’t stop there. The whole thing is mounted on this incredibly complex equatorial mount, built by the Warner & Swasey Company. This mount is what allows the telescope to smoothly track stars as they move across the sky. And to top it all off, the entire setup is housed inside a 90-foot diameter dome, complete with a movable floor that can be raised or lowered so astronomers can comfortably reach the eyepiece. It’s seriously impressive!

The End of an Era? Why Refractors Aren’t King Anymore

So, why aren’t we still building giant refracting telescopes? Well, the Yerkes refractor pretty much represents the limit of what’s possible with that technology. As lenses get bigger, they become incredibly difficult and expensive to make perfectly. And even more importantly, the sheer weight of the glass starts to become a problem. Imagine holding a giant, heavy lens only by its edges – it would start to sag and distort the image. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces .

That’s why astronomers eventually switched to reflecting telescopes, which use mirrors instead of lenses. Mirrors can be supported from behind, which means you can build much larger telescopes without worrying about sagging. Reflecting telescopes have superseded the refracting telescope, which allows larger apertures .

More Than Just a Telescope: A Legacy

Even though it’s over a century old, the Yerkes 40-inch refractor is still a working instrument, and it’s still capable of capturing stunning views of the cosmos. The observatory was a hub of astronomical research until fairly recently, 2018. Now, Yerkes Observatory is focused on education and sharing the wonders of the universe with the public.

While there have been a few larger lenses created since then, like the 43-inch lens used in the Swedish 1-m Solar Telescope, these are mostly for specialized solar work and don’t have the same clear aperture as the Yerkes refractor. The Swedish Solar Telescope has a 39-inch clear aperture . And while the Great Paris Exhibition Telescope of 1900 had a bigger lens, it was more of a showpiece and was eventually taken apart. So, the 40-inch Yerkes refractor still proudly wears the crown as the largest refracting telescope actively used for astronomical research.

The Yerkes Observatory and its Great Refractor are more than just a telescope; they’re a symbol of a time when astronomers were pushing the boundaries of what was possible with glass and ingenuity. It’s a reminder that sometimes, the most amazing discoveries come from simply trying to build something bigger and better.