Star Points: From 'Leviathan' to Gran Telescopio, a history of the world's largest telescopes

One hundred years ago, the largest telescope in the world was the historic 100-inch-diameter (2.5 meter) John D. Hooker reflecting telescope at Mt. Wilson Observatory, just north of the greater Los Angeles metropolitan area. Completed in 1917, the Hooker broke the previous record held since the 72-inch telescope, known as “The Leviathan” was installed at Ireland’s Birr Castle in 1845.

Following World War II in 1948 the 200-inch (5 meter) Hale telescope went into operation at Mt. Palomar Observatory in southern California. Although twice the aperture of the Hooker telescope, the Hale mirror had four times the surface area (i.e. four times the light gathering ability) of Hooker. The Hale remained the largest telescope in the world for 27 years.


The first poured glass mirror for Palomar cracked during cooling. That defective mirror is on display at the Corning Glass Museum in New York. Manufacturing of the second mirror was successful.

The world record holder switched from the United States to the Soviet Union in 1975. The 238-inch (6 meter) BT-4 telescope is nestled in the Caucasus mountain region between the Black and Caspian Seas. As with the Hale telescope, the first mirror manufactured for the BT-4 was a failure due to cracks and bubbles. A second mirror was made but was said to have rough spots and other imperfections that required entire sections of the mirror to be blocked off using black cloth. Almost from the start rumors of technical glitches with the telescope leaked out from behind the secretive Communist veil.

Problems caused by uneven cooling when pouring large molten monolithic mirrors forced future technological changes for large aperture telescopes. Segmented mirrors became the norm in which multiple smaller mirrors (typically hexagonal in shape) were manufactured separately and then arranged together forming a larger mirror. The next record breaker was such a mirror.

The Keck telescope opened at Mauna Kea Observatory in 1993 and sits upon a mountain top on the big island of Hawaii. It’s 394-inch mirror (10 meter) mirror consists of 36 hexagonal segments. Significant weight savings was realized by using thin individual segments instead of a single large thick monolithic mirror. With the Keck the US once again became the world record holder for having the largest aperture telescope.

The US remained the aperture record holder until 2009 when a new telescope opened on one of Spain’s Canary Islands. The Gran Telescopio Canarias (GTC) has an aperture of 410 inches, twice the aperture and four times the light gathering ability of the venerable and historic Mt. Palomar telescope.

Although the GTC is currently the world’s largest, there are projects underway to construct ever larger instruments. One such behemoth is the Giant Magellan Telescope (GMT) located in the Chilean Andes. It, too, is a segmented telescope consisting of seven 27-foot (8.4 meter) mirrors whose combined light will be the equivalent of a single telescope 70 feet (24.5 meters) across. It is anticipated to go online in the mid-2020s.

Local events

Spring is in full swing and the Westminster Astronomical Society (WASI) has planned numerous events for the month. Three events at area nature centers and two at a local vineyard and brewery.

Nature center events are (1) June 8: 7:30 p.m.: Bear Branch Nature Center (BBNC) planetarium program followed by 8:30 p.m. telescopic observing in the B.F. Roelke Memorial Observatory, address 300 John Owings Road; (2) June 15 from 7:30-10 p.m.: Star gazing at Charlotte’s Quest Nature Center, 3400 Wilhelm Lane in Manchester; and (3) July 6 from 8-10 p.m.: Rain or shine program followed by observing at Soldiers Delight Environmental Area, 5100 Deer Park Road in Owings Mills. All events are free except for the BBNC planetarium program. Reserve your planetarium seat by calling 410-386-2103.

Two happy hour star party events featuring solar and lunar viewing are (4) June 7, 5:30-8:30 p.m. at Black Ankle Vineyards, address 14463 Black Ankle Road at Mount Airy: and (5) June 14, 5-9 p.m. at Milkhouse Brewery, address 8253 Dollyhyde Road at Mount Airy near Libertytown.

All programs or portions of the above events involving actual telescopic observing are weather permitting. For additional information on these and other events see the online calendar at