With each successive generation of telescopes, astronomers have been able to see farther into space and study the universe in greater detail. The exact origin of the optical telescope remains uncertain, although its development in Europe at the beginning of the 17th century transformed astronomy. A deeper look at its early history can be found in Ancient Telescopes and Lenses.
Galileo did not invent the telescope, but his astronomical observations demonstrated just how powerful the new instrument could be. From Galileo’s small refractors to today’s enormous segmented-mirror observatories, the following telescopes represent some of the most important steps in the development of observational astronomy.
Galileo’s Refracting Telescope
Galileo Galilei (1564–1642) was not the first person to build or use a telescope, but in 1609 he began improving the recently invented instrument and turning it toward the night sky.
His earliest telescopes offered only modest magnification, but he soon constructed instruments capable of magnifying around 20 times. With them he observed mountains and craters on the Moon, discovered four moons orbiting Jupiter, studied sunspots and saw the phases of Venus. He also observed Saturn’s unusual appearance, although his telescope was not powerful enough to reveal the planet’s rings clearly.
These observations transformed astronomy. In particular, Jupiter’s moons showed that not everything orbited Earth, while the full set of phases displayed by Venus contradicted the traditional Ptolemaic model. The evidence strongly supported the emerging heliocentric view associated with Nicolaus Copernicus.
Newton’s Reflecting Telescope
Isaac Newton (1642–1727) introduced one of the most important changes in telescope design: using a mirror rather than a large objective lens to collect and focus light.
Early refracting telescopes suffered badly from chromatic aberration because lenses bend different wavelengths of light by slightly different amounts. Newton realized that a curved mirror could form an image without producing this colored fringing, and in 1668 he constructed a small working reflector.
In a Newtonian reflecting telescope, a concave primary mirror gathers the incoming light and a small diagonal secondary mirror redirects it toward an eyepiece at the side of the tube. Modern reflectors use much more sophisticated optical designs and coatings, but Newton’s basic principle opened the way to increasingly large astronomical telescopes.
Reflecting designs eventually became dominant in professional astronomy because large mirrors can be supported from behind and constructed at sizes that would be extremely difficult to achieve with lenses.
William Herschel’s 40-Foot Reflecting Telescope
William Herschel’s enormous 40-foot reflecting telescope was constructed at Slough, England, between 1785 and 1789. Its name referred to its approximately 40-foot (12-meter) focal length, while its primary mirror measured about 48 inches (1.2 meters) across.
The telescope became one of the most famous scientific instruments of the 18th century. Herschel used it in 1789 when he discovered two moons of Saturn, Mimas and Enceladus, although much of his highly productive observing career relied on smaller and more manageable telescopes.
The 40-foot telescope nevertheless demonstrated how far reflecting instruments could be scaled up. It remained a celebrated symbol of astronomical engineering long after it became impractical to maintain. The structure was dismantled in 1840, although parts of the original instrument survive.
The Leviathan of Parsonstown
In the 1840s, William Parsons, the 3rd Earl of Rosse, constructed an even larger reflector at Birr Castle in Ireland. Its six-foot (1.8-meter) metal mirror made the Leviathan of Parsonstown the world’s largest telescope when observations began in 1845.
Its most famous achievement came that same year, when Lord Rosse observed a distinct spiral structure in M51, the Whirlpool Galaxy. At the time, objects such as M51 were still generally described as nebulae, and their true nature lay at the heart of a major astronomical debate.
The Leviathan did not establish that spiral nebulae were separate galaxies; that breakthrough would come much later. What it did reveal was that some nebulae possessed organized spiral structures, providing astronomers with an entirely new class of object to investigate.
The telescope remained the world’s largest by aperture for more than seven decades and was also used extensively in the study of nebulae and star clusters.
The Hooker Telescope
The 100-inch (2.54-meter) Hooker Telescope at Mount Wilson Observatory in California entered service in 1917 and became one of the most scientifically important telescopes of the 20th century.
Edwin Hubble used observations made with the Hooker to identify Cepheid variable stars in the Andromeda Nebula. Their distances showed that Andromeda lay far beyond the Milky Way, establishing that our galaxy was only one among many.
Hubble and Milton Humason subsequently investigated the relationship between the distances of galaxies and their redshifts. Their work helped establish the observational evidence for an expanding universe and fundamentally changed our picture of the cosmos.
The Hooker also contributed to Walter Baade’s studies of stellar populations and Cepheid variables, which led to a major revision of the extragalactic distance scale. The telescope therefore played a central role in transforming astronomy from the study of the Milky Way and nearby stars into the modern science of galaxies and cosmology.
The Hale Telescope
The 200-inch (5.1-meter) Hale Telescope at Palomar Observatory in California effectively doubled the aperture of the Hooker Telescope when it entered service in 1949.
Creating a mirror of this size required major advances in telescope engineering. Its primary mirror was made from low-expansion Pyrex glass and coated with a reflective layer of aluminum. The telescope’s enormous equatorial mounting allowed the 200-inch mirror to track celestial objects accurately across the sky.
For decades, the Hale was one of astronomy’s premier research instruments and was used to study everything from stars and galaxies to quasars and the expansion of the universe. Its five-meter mirror also demonstrated that very large reflectors could operate successfully as precision scientific instruments.
The Hale remained the world’s largest effective optical telescope for decades and is still used for astronomical research today.
BTA-6
The Soviet BTA-6 telescope in the Caucasus Mountains took the next step in aperture. Its six-meter (19.7-foot) primary mirror made it the world’s largest optical telescope when it began operations in the 1970s.
The BTA-6 was also important because it abandoned the enormous equatorial mount traditionally used by large telescopes in favor of an altitude-azimuth design. Modern computer control made this type of mounting increasingly practical, and it subsequently became standard for most very large observatories.
The telescope suffered significant problems, including difficulties with its original primary mirror and observing conditions affected by atmospheric turbulence. Its performance therefore never fulfilled all of the ambitions surrounding its construction.
Nevertheless, the BTA-6 remained scientifically useful, particularly for spectroscopy and other observations that benefit from its large light-gathering area. Just as importantly, its engineering helped demonstrate both the possibilities and the difficulties of pushing single-mirror telescopes beyond the five-meter scale.
Keck I and Keck II
The twin Keck telescopes on Mauna Kea, Hawaii, showed how segmented mirrors could overcome the practical limits imposed by a single enormous piece of glass.
Each telescope has an effective aperture of 10 meters (33 feet), with its primary mirror constructed from 36 hexagonal segments. A computer-controlled active-optics system continually maintains the precise shape and alignment of the segmented primary mirror as the telescope changes orientation.
Keck I began scientific observations in the 1990s and was followed by Keck II. The observatory has since contributed to research across an enormous range of astronomy, including exoplanets, the center of the Milky Way, distant galaxies and the early universe.
The success of Keck also demonstrated that segmented mirrors could function as a single giant optical surface. That principle now underpins an even larger generation of ground-based telescopes.
Hubble Space Telescope
Launched in 1990, the Hubble Space Telescope transformed astronomy by placing a 2.4-meter telescope above the blurring effects of Earth’s atmosphere.
Hubble has contributed to almost every major field of modern astronomy. Its observations helped refine measurements of the expansion rate of the universe, revealed the structures of galaxies and nebulae in extraordinary detail, studied the atmospheres of exoplanets, and produced the famous Hubble Deep Field and its successors, exposing thousands of extremely distant galaxies.
More than 36 years after launch, Hubble remains scientifically active in 2026. Its ability to observe ultraviolet and visible wavelengths is particularly valuable and complements the infrared capabilities of the James Webb Space Telescope.
Webb therefore did not simply replace Hubble. The two observatories study overlapping but different parts of the electromagnetic spectrum and can combine their observations to provide a broader view of astronomical objects.
The Extremely Large Telescope
The next leap in ground-based optical astronomy is already taking shape on Cerro Armazones in Chile’s Atacama Desert. The European Southern Observatory’s Extremely Large Telescope (ELT) is designed around a segmented primary mirror 39 meters (128 feet) in diameter.
Its primary mirror will consist of 798 hexagonal segments working together as one optical surface. The ELT uses a five-mirror optical design, including adaptive optics that will compensate for atmospheric distortion and allow the telescope to achieve exceptionally sharp observations in visible and near-infrared light.
The enormous collecting area will allow astronomers to study extremely faint and distant objects while also resolving fine detail that is beyond the reach of today’s ground-based observatories. Among its major scientific goals are characterizing planets around other stars, studying the formation and evolution of galaxies, investigating supermassive black holes, and probing some of the earliest structures in the universe.
The ELT remains under construction in 2026. ESO currently plans telescope first light for 2029, followed by the installation and commissioning of its first scientific instruments. The first scientific observations are expected in late 2030.