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5 of the World's Largest Telescopes—and Their Discoveries

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Galileo Galilei didn't invent the telescope, but he did create one that magnified objects about 30 times. One night in 1610, he aimed it at Jupiter—and in the process launched a new era of astronomy. We’ve come a long way since then. The 1930s ushered in telescopes with mirrors more than six feet across, and in 1948 a telescope with an almost 17-foot mirror was unveiled in California. More recently, telescope sizes have expanded to 30 feet and beyond, and the next generation of giant telescopes under construction will exceed 80 feet. The bigger the telescope, the farther and more clearly astronomers can see into space. Here are 5 of the largest optical telescopes in the world, along with significant discoveries made at each one. You can visit all of them.    


Benjamín Núñez González, Wikimedia Commons // CC BY-SA 4.0

Located in La Palma, on the Canary Islands, this 10.4 meter or 34-foot telescope, currently the world’s largest, is a Spanish initiative led by the Instituto de Astrofísica de Canarias. The project also involves Mexico’s Instituto de Astronomía de la Universidad Nacional Autónoma de México, Instituto Nacional de Astrofísica, and Óptica y Electrónica, and the University of Florida.

Recently, the GTC participated in identifying microquasar M81 ULS-1, an "ultraluminous source" in the spiral galaxy M81. A microquasar is a massive star paired with a compact star or black hole; the latter has an accretion disk composed of material swirling around it and an intense, variable radio emission. This emission is normally in the form of symmetric jets of matter shooting out in opposite directions. What makes M81 ULS-1 interesting is that the ejected material approaches the speed of light. Only one other microquasar has been discovered with this characteristic (SS433, found in 1979 within the Milky Way). At only some 13 million light years from the Milky Way, its host galaxy, M81, a seventh magnitude object, can be observed with binoculars.

Guided tours include Observatory facilities and the interior of a telescope (which one depends upon availability) along with details on how it works.


These two 33-foot (10 meter) telescopes dominate the Keck Observatory at 14,000 feet atop Mauna Kea on the island of Hawaii. The first laser guide star adaptive optics system on a large telescope was commissioned on the Keck II in 2004 and helped reveal the black hole at the center of the Milky Way—one of the most significant discoveries in the field of astronomy. More recently, the Keck Observatory helped discover a distant massive galaxy cluster with a core bursting with new stars. SpARCS1049+56 is forming stars at the astonishing rate of more than 800 solar masses per year—800 times faster than in our Milky Way.

Adaptive optics corrects for turbulence in the Earth’s atmosphere using hundreds of actuators that change the shape of deformable mirrors at a rate of 2000 times per second, providing near-perfect detail for planets, stars, and galaxies.

Mauna Kea has a visitor center at 9200 feet with telescopes and guides available. The summit, accessible only by 4-wheel drive, is open from a half-hour before sunrise to a half-hour after sunset.


Part of the South African Astronomical Observatory, this telescope is the largest in the Southern Hemisphere, with a hexagonal mirror array 36 feet or 11 meters across. It is located at an altitude of 5,770 feet in a remote area of the Northern Cape Province and run by a consortium of international partners from South Africa, the United States, Germany, Poland, India, the United Kingdom and New Zealand.

Astronomers here recently discovered a supermassive black hole in the center of galaxy SAGE0536AGN. Black holes are found in most galaxies, but this one is notable for its size: 30 times more massive than would be expected for a galaxy this size. The black hole’s mass is 350 million times that of our Sun, making it a hundred times more massive than the one in the center of the Milky Way, while the galaxy itself has less mass than our galaxy.

Guided tours of the observatory include exhibits on the radio spectrum (SALT identifies individual stars by the light they emit) and a look at the telescope’s 11 enormous, hexagonal mirrors.


Located at the University of Texas McDonald Observatory in Fort Davis, Texas, this recently refurbished 30-foot telescope is the world's third largest optical telescope and most powerful wide-field spectroscopic one. Astronomers used it in 2012 to measure the most massive black hole ever discovered—the size of 17 billion Suns—in galaxy NGC 1277. Typically, a black hole makes up about 0.1 percent of the mass of its host galaxy, but this one accounts for 14 percent of its galaxy's mass. This and similar discoveries in other galaxies could change current thinking about how black holes and galaxies form and evolve.

There is a visitor center, daily tours of the large telescopes, and star parties three nights a week.


Located at Paranal Observatory, part of the European Southern Observatory operations in Chile, the Very Large Telescope array consists of four unit telescopes, each 27 feet or 8.2 meters across, and four auxiliary telescopes 6 feet or 1.8 meters wide, that work together to form the ESO Very Large Telescope Interferometer. It is capable of observing objects four billion times fainter than what can be seen with the naked eye—equivalent to seeing the headlights of a car on the Moon. Among the VLT’s notable discoveries are the first image of an extrasolar planet, tracking of individual stars circling the black hole at the center of the Milky Way, and observing the afterglow of the furthest known gamma-ray burst.

Recently, the VLT recorded details of the spectacular aftermath of a cosmic collision that happened 360 million years ago. Within the resulting debris, images revealed a rare and mysterious young dwarf galaxy, NGC 5291. Dwarf galaxies such as this one are expected to be common in the early universe but are normally too faint and distant to be observed.

Guided tours generally take place every Saturday between 9 a.m. and 2 p.m.

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This Newspaper Article Was Hyping the 2017 Eclipse All the Way Back in 1932
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If you’ve turned on a news station or browsed the internet recently, you’ve likely learned of the total solar eclipse set to pass over the U.S. on Monday, August 21. Many outlets (Mental Floss included) have been talking up the event for months, but the earliest instance of hype surrounding the 2017 eclipse may have come from The New York Times.

Meteorologist Joe Rao presented this news clip at a recent panel on the solar eclipse at the American Museum of Natural History, and fuel analyst Patrick DeHaan shared the image on Twitter earlier this year. It shows a New York Times article from August 1932, selling that year’s eclipse by saying it will be the "best until Aug. 21, 2017."

The total solar eclipse on August 21 won’t be the first to fall over U.S. soil in 85 years. The next one to follow the 1932 eclipse came in 1970, but an author at the time apparently predicted that "poor skies" would be likely for that date. That early forecast turned out to be correct: There were clouds over much of the path of totality in the southeastern U.S. The next total eclipse visible from America, which the article doesn’t mention, happened in 1979. Overcast skies were a problem for at least some of the people trying to view it that time around as well.

The upcoming total eclipse will hopefully be worth the decades of hype. Unlike the previous three, which only skimmed small sections of the lower 48 states, this next eclipse will be visible throughout day as it travels from coast to coast. Check out our field guide for preparing for the once-in-a-lifetime phenomenon.

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Big Questions
Can You Really Go Blind Staring at a Solar Eclipse?
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A total solar eclipse will cut a path of totality across the United States on August 21, and eclipse mania is gripping the country. Should the wide-eyed and unprotected hazard a peek at this rare phenomenon?

NASA doesn't advise it. The truth is, a quick glance at a solar eclipse won't leave you blind. But you're not doing your peepers any favors. As NASA explains, even when 99 percent of the sun's surface is covered, the 1 percent that sneaks out around the edges is enough to damage the rod and cone cells in your retinas. As this light and radiation flood into the eye, the retina becomes trapped in a sort of solar cooker that scorches its tissue. And because your retinas don't have any pain receptors, your eyes have no way of warning you to stop.

The good news for astronomy enthusiasts is that there are ways to safely view a solar eclipse. A pair of NASA-approved eclipse glasses will block the retina-frying rays, but sunglasses or any other kind of smoked lenses cannot. (The editors at, an eclipse watchers' fan site, put shades in the "eye suicide" category.) NASA also suggests watching the eclipse indirectly through a pinhole projector, or through binoculars or a telescope fitted with special solar filters.

While it's safe to take a quick, unfiltered peek at the sun in the brief totality of a total solar eclipse, doing so during the partial phases—when the Moon is not completely covering the Sun—is much riskier.


NASA's website tackled this question. Their short answer: that could ruin their lives.

"A student who heeds warnings from teachers and other authorities not to view the eclipse because of the danger to vision, and learns later that other students did see it safely, may feel cheated out of the experience. Having now learned that the authority figure was wrong on one occasion, how is this student going to react when other health-related advice about drugs, alcohol, AIDS, or smoking is given[?]"

This story was originally published in 2012.


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