The first telescopes, known as refractive telescopes, were built by Dutch spectacle makers in the early 17th century. They used a pair of lenses: one convex at the end of the telescope and one concave in front of the eyepiece. For the most part, these telescopes were used to survey land and for military exploits. The Italian astronomer Galileo Galilei was one of the first to point such binoculars at the sky.
Johannes Kepler, a German astronomer, improved the convex-concave design with a pair of convex lenses. The advantage was a larger field of view and higher magnification, but the images appeared upside down. Still, telescope makers who implemented Kepler’s design were able to achieve 100 times magnification with telescopes as long as 150 feet. However, such long tubes did not perform well in all weather conditions, making them somewhat ineffective.
Sir Isaac Newton offered an alternative design based on reflectionor curved mirrors, which collect more light and avoid the distorting prism effect that occurs when light passes through a lens, known as chromatic aberration.
For more than two centuries, the size, material and quality of mirrors continued to increase – as did the size of telescopes, until the Space Age when space-based telescopes such as Hubble And James Webb, eliminated the interference from Earth’s atmosphere. With James Webb we can see further through the universe than ever before, spying on some of the first galaxies emerged after the Big Bang, over 13 billion years ago.
Today, NASA has several space telescope projects underway, including the Nancy Grace Roman telescope and Habitable worlds observatory.
1609: Refractive telescopes
Inspired by Dutch and Danish telescope makers, Galileo built his own telescope in 1609. His first telescope offered a magnification of 3x. Over the years his design continued to improve. His last telescope could magnify objects as much as 30 times.
Fortunately, Galileo was not only a gifted astronomer but also a skilled artist, which allowed him to create detailed images of the cosmic objects he spotted through his lens. This sketch of the moon revealed never-before seen – or contemplated – lunar mountains and craters.
1672: Sir Isaac Newton’s reflecting telescope
When light passes through glass, it is separated into bands of color (ROYGBIV), which meant that refracting telescopes suffered from chromatic aberrations that affected image quality. In an attempt to overcome the prism effect, Sir Isaac Newton built a reflecting telescope that used curved mirrors instead.
Laurent Cassegrain Improved on Newton’s design in 1672 by using a concave primary mirror and a convex secondary mirror to reflect light back through a hole in the primary mirror to the eyepiece, allowing a long focal length in a compact tube.
1789: Herschelian telescope
Sir William Herschel’s telescope had a reflective design with a large primary mirror and an eyepiece positioned off-axis so as not to obstruct the light path. This allowed for larger mirrors and greater light gathering power.
With one of his telescopes, Herschel spied a new planet, which he named Georgium Sidus for King George III. The planet was later named Uranus.
Herschel’s drawings of nebulae from “The Scientific Papers of Sir William Herschel”, published in London in 1912 by the Royal Society and the Royal Astronomy Society.
1900: The large telescope of the Paris exhibition
This 57 meter reflecting telescope – more than half the length of a football field – was built by Paul Gautier for the Paris Exposition of 1900. It had a mirror with a diameter of 1.25 meters.
1917: Hooker telescope
The Mount Wilson Observatory’s Hooker Telescope, designed by George Ellery Hale, used a 100-inch diameter mirror, making it the largest telescope in the world at the time. It has significantly advanced the study of galaxies and nebulae.
Edwin Hubble used the Hooker telescope in the 1920s and later, which paved the way for our understanding that the universe was much bigger than just our galaxy, and the Big Bang theory.
1990: Hubble Space Telescope
Built by NASA and the European Space Agency (ESA), the Hubble Space Telescope is a space-based reflecting telescope with a mirror with a diameter of 2.4 meters. It operates in the Earth’s orbit above the atmosphere. When it came into use, it provided unprecedented clarity and detail in its observations of the universe.
NASA expects the telescope to remain operational until the end of 2020.
2021: James Webb Space Telescope
A collaboration between NASA, ESA and the Canadian Space Agency (CSA), the James Webb Space Telescope is an advanced space-based reflecting telescope with a segmented mirror with a diameter of 6.5 meters.
It operates in the infrared spectrum, allowing it to observe distant galaxies, exoplanets and other celestial phenomena with exceptional sensitivity.
The Webb Telescope orbits the Sun near the second Sun-Earth Lagrange point (L2), one million miles from Earth.
