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Sometimes looking into a pool of a toxic liquid holds the secrets of the universe–or maybe just this one time.

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In 1909, the American astronomer Robert Wood published a report on his new experimental telescope that some of his contemporaries straight-up called a joke.

It did not use a big lens to collect light, or the standard glassy mirror; no, when the universe's light rays came streaming in, the thing that they encountered was a pool of liquid mercury.

Wood had designed a Liquid Mirror Telescope, and while he quickly abandoned the idea, modern astronomers have returned to the design as a legitimate albeit quirky addition to their ground telescope fleet.

So, to study the cosmos, astronomers capture light. Sounds simple enough. But those light rays have to be captured in just the right way so the resulting image comes out clear.

And ever since Isaac Newton came along in the seventeenth century, the most popular way to do this has been with curved mirrors.

But not just any curve. It's a specific shape called a parabola, which focuses all the light rays that come in at any angle onto a single point.

As for the size, bigger is better. The bigger the mirror, the more light it can capture and the more detail we can make out in the images it produces.

Over the centuries, telescope mirrors started getting too big-- not for the science, but for engineering. Large mirrors are difficult to transport, they're fragile, they're incredibly expensive.

Oh, and after you make them, you have to continue polishing them throughout their lives so they can keep doing their job. That's why super large telescopes often make their mirrors in segments and then piece them together like a jigsaw puzzle.

But, in the mid-1800s, a few astronomers considered a different way to collect light without breaking the bank.

Instead of, like, freezing the shape of a parabola into a solid piece of glass or shiny metal, a Liquid Mirror Telescope, or LMT, would create its curve with the power of physics.

All you need to do is fill a shallow container with the liquid of your choice and spin it. As the container spins, the centripetal force will, you know, force the edge of the liquid to flow up as it presses against the walls of the container.

Meanwhile, the center of the pool is pulled downward by gravity. Boom! It's a parabola! And by changing the speed of rotation you can adjust how extreme the curve of that parabola is and how far away all the light rays will meet up. The faster it spins, the more deeply curved you can make your telescope's reflector.

So the first of many challenges in building an LMT was finding the right liquid. Now, water reflects sunlight-- just ask Narcissus-- but not well enough for astronomers to study the night sky.

So the first known LMT experiments used an even more reflective liquid: mercury, even though scientists of course already knew that hanging out around it was bad for your health.

In 1875, down at the Dunedin Observatory in New Zealand, Henry Skey built a proof of concept liquid mirror that was 35 centimeters wide. He produced test images in the laboratory but never used it for actual observations.

Robert Wood, however, aimed bigger: 50 centimeters, or half a meter, and it worked! Unlike Skey before him, Wood actually used his LMT to observe the night sky.

He was able to clearly see the Milky Way and track a few individual stars as they passed directly overhead. Unfortunately, his LMT also suffered from some serious setbacks.

The mercury was incredibly susceptible to environmental disturbances, such as the occasional horse-drawn carriage rolling by, that would create ripples in the basin and distort the image.

And that's not all. The machine Wood used to spin his liquid mirror couldn't keep the speed perfectly constant, and those small changes would introduce ripples too.

Now, with enough effort and perhaps experimental technology, Wood might have diminished those distortions, but he stopped his LMT research altogether because he saw an even bigger problem, one that he could never fix.

Because the shape of a liquid mirror relies on gravity, an LMT can only point straight up, and as Wood saw it, if your telescope can only point straight up, you are super limited in what you can study.

But, in 1909, there was a lot of stuff out there in the universe that astronomers had no idea even existed. When Wood was developing his LMT, he didn't know that those fuzzy spiral nebulas like Andromeda were other galaxies, or that the universe was full of galaxies no matter where you look, including straight up!

There are telescopes that do just that. They're called Zenith Telescopes. And over the course of a single night, they view very narrow bands of the sky, and over time, they focus on repeated viewings of that same band to collect a large amount of data.

Zenith Telescopes are great for studying things like supernovas, gravitational lensing, and quasars, which are super massive black holes at the center of galaxies that feed on the gas around them.

Basically, if an astronomer needs repeated viewings to study a phenomenon but can't get the extended observing time they'd need on a big general-purpose telescope like the Hubble or JWST, a Zenith Telescope is a good way to go.

After decades of gathering dust, the notion of an LMT reappeared in the 1980s as a cost-efficient option for building a massive, next-gen ground-based telescope.

For example, the 6 meter large Zenith Telescope in British Columbia, Canada, which ran from 2003 to 2016, had a price tag of only half a million dollars.

That might sound like a lot for a tub of toxic metal, but compared to a solid mirror of the same size, it's less than one tenth the cost. And don't worry, they did have safety standards, and apparently, no one got sick.

The LZT overcame Wood's ripples by using techniques that weren't available to Wood at the time, like using compressed gas to create a cushion of air that minimized friction as the liquid pool spun around.

And by putting a layer of glycerin or other clear liquid on top of the mercury, the team was able to dampen any remaining ripples caused by the environment or by machinery.

If the liquid mirror got dust on it, messing up the images, it could be drained and filtered, and that was way easier and faster than having to polish a bunch of solid mirrors over and over.

While its cloudy Canadian locale made it not particularly great for night sky observing, the LZT was successful enough that Liquid Mirror Telescopes are here to stay. Based in the Indian Himalayas, the 4 meter International Liquid Mirror Telescope saw its first light in 2022.

Robert Wood had no idea that his telescope would be used to view exploding stars or galaxies at the end of the universe. But he took an idea that some considered a joke and actually tried to make it work.

His telescope was not without its flaws, but hey, it's kind of scientists' thing to stand on the shoulders of giants and improve on what came before.

And you might say that reflections of Wood's work can still be seen in every Liquid Mirror Telescope that has come after.