The W. M. Keck Observatory from another angle with its laser guide visible. Source

An inside look of one of the domes, showing the primary mirror for the Keck I. Source

The Hobby-Eberly Telescope, located at the McDonald Observatory in West Texas, is one of three telescopes at the facility and is the largest since being built in 1996. The HET is different from a lot of telescopes around the world in that its massive mirror remains stationary, while the other components maneuver across it. The telescope is limited in what it can track, and can only watch a single object for about two hours continuously. This unique construction drastically cut down on costs, and with a 30-foot aperture it's still one of the world's largest 'scopes. Source

A second view of the Hobby-Eberly Telescope with its dome open. The HET uses a segmented mirror broken into 91 hexagonal parts, similar to the Keck telescopes. Source

The Navy Prototype Optical Interferometer just outside of Flagstaff, Arizona is one of the world's few optical interferometers, and rivaled in power only by the new CHARA Array in California even though the NPOI has been active since '96. It gets amazingly accurate results thanks to its use of up to six mirrors rather than just one, as well as, according to the Lowell Observatory (which helps run the NPOI along with the Navy), a method that involves "light that can be best gathered and directed with mirrors and lenses, generally from the visible light through near infrared light." This information helps scientists establish an interference pattern, which can be used to study celestial objects in detail. Source

A detailed diagram of the Navy Prototype Optical Interferometer. Source

Active since 1998, the four optical telescopes you see here — named Antu, Kueyen, Melipal and Yepun — make up what's known collectively as the Very Large Telescope located in the Atacama desert in northern Chile. Each one has an aperture rivaling the HET at nearly 27 meters, and when working together they're often used to resolve small objects, such as distant galaxies and even evidence for a black hole at the center of our universe. It was that in 2004 that the VLT had spotted the most distant galaxy ever observed, Abell 1835, though researchers could not corroborate the results using orbital telescopes, and the VLT didn't score the honor. Source

A close up of one of the four optical telescopes that make up the Very Large Telescope in Chile. Source

The baby on the block, the Gran Telescopio Canarias (or "Canaries Great Telescope") has only been active since 2009 and is currently the world's largest single-aperture optical telescope with a record-setting aperture of 34 feet. Located on Spanish La Palma in the Canary Islands, the telescope was completed in 2008 and built at an altitude of nearly 7,500 feet. Time on the GTC is enormously expensive, and the University of Florida, which owns a 5‰ share of the telescope, only gets a total of 20 nights per year to use the GTC. The University secured 35 additional nights by donating the CanariCam, a mid-range infrared imager. Source

A view of the inner workings of the Gran Telescopio Canarias, the largest single-aperture optical telescope in the world. Source

Situated on the 10,700-foot-tall Mount Graham in Arizona, the Large Binocular Telescope knows, like the Keck Observatory, that two are better than one. It sports a pair of 28-foot mirrors that, working together, can operate in an interferometric mode much like the Navy Prototype Optical Interferometer. The LBT can also operate in an aperture synthesis mode, where it works with an effective single surface of 39 feet — larger than even the Gran Telescopio Canarias. It's been gazing at the stars since 2005, though even in that short of a time it's been through some hardship: namely, from lawsuits over an endangered America Red Squirrel population and two raging forest fires, neither of which harmed the facility, luckily. Source

The dual mirrors at the heart of Arizona's Large Binocular Telescope. Source

Here's that top shot of the Large Binocular Telescope so you can see it in all its glory, and it's a great one to wrap up our gallery with — especially at night, it's hands-down the most striking telescope in the world. Source

Built in 1993 atop the summit of Mauna Kea in Hawaii at an altitude of nearly 13,600 feet, the W. M. Keck Observatory pioneered the use of robotic assistance to make telescopes more accurate thanks to rapid, minute adjustments within the span of a second. At 33 feet in diameter each, the Keck I and Keck II telescopes are among the largest in the world, second only to the Gran Telescopio Canarias (which you'll see in this gallery). Source