Really Big Telescopes

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Since the dawn of time, humans have looked up at the night sky to watch the stars.

…and then nothing happened for hundreds of thousands of years until a guy by the name of Galileo Galilei point a telescope at the stars and saw a bunch of stuff that everyone had missed.

Since then, we’ve increased the size of our telescopes so we can see more and more, further and further away.

Learn more about the ever-increasing size of astronomical telescopes on this episode of Everything Everywhere Daily.

On January 7, 1610, Galileo picked up a telescope and pointed it at the planet Jupiter, it marked a major turning point in human history. This was the moment when astronomy was separated from astrology. 

Prior to this moment, humans were more concerned with grouping together specks of light and following their path in the sky, than they were with trying to figure out what those specks of light were. In fact, most people never even really bothered to think that those specks of light might be anything other than specks of light. 

The telescope which was used by Galileo was nothing more than what you might find in a toy store, with a magnification on par with a decent pair of binoculars. 

With this very simple tool, he was able to immediately discover things that we all take for granted but were never known in human history beforehand. 

He discovered that Jupiter had moons, that Venus has phases, that the moon had mountains, that the Milky Way was made up of stars, that Saturn had rings, and that the sun had sunspots and that it rotated.

Once these discoveries were made public, the race was on to create larger and more powerful telescopes. 

Here I should explain the basic principle of telescopes. 

The power of a telescope is based on the aperture or the diameter of the lens. The larger the lens, the more light can be gathered. If you think of photons light as drops of water streaming down, and a telescope as a bucket, then the bigger the bucket, the more photons you’ll capture. 

A telescope with a glass lens you look through is called a refracting telescope. 

As a lens gets larger wider, the area of the lens will grow in size corresponding to the square of the radius. 

Galileo’s first astronomical telescope had an aperture of 1.5 centimeters.  Within 10 years, the aperture of his telescopes had almost tripled in size to 3.8 centimeters. 

By the end of the 17th century, Dutch astronomer Christiaan Huygens was using a telescope with an aperture of 22 centimeters. With this, he was able to see Saturn’s largest moon, Titan. 

There was a big problem with these telescopes. As they got larger, it became more and more difficult, and more and more expensive to create the lenses. 

Glass is heavy. As lenses get larger, they require longer focal lengths, which means larger tubes. Eventually, astronomers had to create small buildings to house their telescopes. Also, they need very heavy mounts to hold such a large heavy object in place. Finally, crafting and grinding such large lenses at this scale was very hard to do. 

Refracting telescopes did increase in size through the 19th century, but eventually, they hit a limit. The largest refracting astronomical telescope ever made was built in 1897 at the Yerkes Observatory in Wisconsin. It has an aperture of 104 centimeters or 40 inches. Today it is mostly a museum piece. 

There was a larger refracting telescope made for the 1900 Great Paris Exhibition. It had an aperture of 125 centimeters, however, it was designed more as a novelty and not as a scientific device. It was pointed horizontally through what looked like an oil pipeline and it had a focal length of 57 meters or 187 feet.

Beyond this 1 meter threshold, refracting telescopes were simply too impractical and expensive to build. 

Thankfully, a guy you might have heard of called Isaac Newton invented another type of telescope back in 1668 called a reflecting telescope. Instead of a glass lens, you looked through, it used a large curved mirror at the bottom of a tube. Light would go into the tube, hit the mirror, and get focused to another mirror back near the opening of the tube, which would bounce the light sideways to an eyepiece. 

The reflecting telescope had a huge advantage over the refracting telescope in that a mirror was far easier, cheaper, and lighter to manufacture than a glass lens. 

By the 18th century, the largest telescopes were all reflecting telescopes and that was where most of the major astronomical research was being done. 

In 1785, English astronomer William Herschel was using a reflecting telescope with a 120-centimeter mirror, and with this, he was able to discover the planet Uranus, as well as Saturn’s 6th and 7th moon.

The entire apparatus to hold up the telescope was enormous and looked like an oil rig. 

The mirrors kept getting bigger. The Hale Telescope at the Mount Wilson Observatory in California was a meter and a half in diameter in 1908, and the Hooker Telescope, also at Mount Wilson, was a 2 and a half meter telescope installed in 1917.

Going back to my analogy of a telescope as a light bucket, the size of the bucket is the biggest factor in how much light you will collect. Another factor was how long you let the bucket fill up with light. The longer you hold a bucket under running water, the more water you will collect, and the same is true with light.

Up until the late 19th century, all telescopes worked by an astronomer putting their eye on an eyepiece and looking at the light as it streamed through the telescope in real-time. 

A major development was the use of cameras.  A camera, assuming that the telescope could move with the rotation of the earth to keep still, could collect light for a much longer period of time. What might look like a blurry smudge to the naked eye, can be a crisp, sharp, and bright image from a camera with a long exposure. 

Astrophotography became the way in which all serious astronomy was conducted. One of the first achievements of astrophotography was the discovery of Pluto. It is believed that astronomers actually did see Pluto before it was known to be a planet. However, it was so far away, and so slow, and no one knew it was moving.

Pluto’s discoverer, Clyde Tombaugh, discovered the object by comparing photographs and noticed that one of the lights had moved. 

Today all professional astronomical telescopes are reflecting telescopes with a camera. In fact, the really big telescopes don’t even have eyepieces. 

Just as large glass lenses had a limit in size, so too did large mirrors. As mirrors became bigger, they became subject to gravity distorting the mirror. The surface of the mirrors had to be ground down to an insanely smooth level of well less than a micron. 

By the 1980s, the big single mirror had run its course. They needed new techniques to make larger telescopes. 

One such technique was adaptive optics. As light comes through the atmosphere, it can get blurred. This is why stars can appear to twinkle. Adaptive optics can make thousands of tiny micro-adjustments to the mirror in real-time to compensate for the changes in the light. 

Segmented mirrors also allowed for larger telescopes. Rather than make one massive mirror, you make many smaller hexagonal mirrors that fit together to make one big mirror.

Finally, you can make multiple telescopes work together through what’s known as interferometry. In 1993, the Keck 1 and Keck 2 telescopes were opened. At the time they were individually the largest telescopes in the world with each telescope having mirrors with a diameter of 10 meters. 

The Keck telescopes used all of the above techniques I mentioned. The main mirrors are segmented, use adaptive optics, and they can be used together via interferometry as one giant 85-meter telescope. 

Since the Keck’s were opened almost 30 years ago, the crown of the largest telescope was taken by the Gran Telescopio Canarias in the Canary Islands which has a 10.4-meter mirror I’ve been able to see both the Keck and Gran Telescopio up close on tours, and it is well worth the time and effort to see them if you ever get the chance. 

These however are not the last of the big telescopes. Progress in the creation of giant mirrors hasn’t stopped. 

Currently, the European Extremely Large Telescope or ELT is under construction in Chile. It will have a 39.3-meter diameter primary mirror, which is almost 4x the diameter of the current largest telescopes. The ELT is expected to help in the search to find Earth-like planets around other stars. It is expected to see first light in 2025.

There are even conceptual plans for something called the OWL, or the Overwhelmingly Large Telescope. This would have a 100-meter mirror. This is still in the planning stages and building a telescope of this size would be an enormous undertaking. However, from an engineering perspective at least, it is feasible. 

All of the biggest telescopes, both present, and future, are all found in only a few places. They are in the Atacama Desert of Chile, on top of Mauna Kea in Hawaii, or on the island of La Palma in the Canary Islands. 

The reason for this is weather. When you build something this big, you want to have as many viewing days as possible. Cloud cover prevents you from seeing anything, so you need a location that is either above the clouds, like Mona Kea, or in a place that has no clouds, like the Atacama. 

While something like the OWL might be possible, even the large reflecting telescopes are running into problems.

Not only do massive telescopes cost billions of dollars, but there are cultural issues with building new telescopes in places like Mona Kea, and we are ultimately limited to the fact that we are on Earth, with all our messy atmosphere and a sun which ruins observations for half of each day.

All of these problems can be solved by going to space. 

In 1990, the Hubble Space Telescope was launched on the Space Shuttle. With a 2.4 meter mirror, it isn’t close to being the largest telescope on Earth, however, it has several advantages over ground-based telescopes. 

Because it is in the vacuum of space, the atmosphere is a non-issue. Also, it can take very long exposures. The Hubble Extreme Deep Field image was a single image taken over the equivalent of a 23-day or 2 million second exposure. 

Even though the mirror is much smaller, because it is in space, the Hubble can get a resolution far better than much larger telescopes on the ground.

Just as technology is making ground-based telescopes larger, so too are there plans for even larger space-based telescopes. 

The James Webb Telescope is almost completed and is expected to be launched sometime in 2021. 

The Webb will be different from the Hubble in several ways.

First, it will not orbit around the Earth. It will be placed in a stable gravity point, called a Lagrange Point, almost four times the distance from the moon. Unlike the Hubble, it will not be constantly zipping around the planet every 95 minutes. This will allow for some very long single exposures. 

Second, the Webb will have a far larger light-collecting area. The Webb’s primary mirror will be 6.5 meters, with over 15 times the light-collecting area.

Finally, the Webb will have a large sun shield the size of a tennis court to block light from the sun. 

You might have noticed that 6.5 meters is far larger than the width of any rocket. The web will basically be folded in the world’s most elaborate and expensive origami. It will take a full 2 weeks for the telescope to slowly unfold and a full month to get to its final place in space. 

All of the telescopes I’ve mentioned are collecting light in the visible or infrared parts of the spectrum. Once you get into radio astronomy, you can have dishes that take up entire volcano calderas like the Arecibo Telescope in Puerto Rico, and with interferometry, you can have telescopes all over the Earth working in unison.

Astronomy is a very exciting field right now. With the new telescopes which will be coming online in the next few years, we should expect discoveries which will rival anything we’ve seen this far.