The telescope is one of those items that people both rarely think about and take entirely for granted. 2008 was the 400th anniversary of its invention in 1608. When Columbus sailed the ocean blue, he didn’t have a spyglass to help him find land. It is younger than the printing press and the Protestant Reformation.

The technology that makes a telescope appears in such everyday items as binoculars, the telephoto lenses in cameras, and even in spy satellites. The planets Uranus and Neptune (among other things) had never even been imagined and would still be unknown were it not for the invention of the telescope. Oddly, we might still imagine that the sun revolves around the Earth instead of the other way around if it weren’t for the telescope—and chances are, there would never have been a space program: so no weather satellites to warn us of hurricanes, no GPS satellites to tell us where to go, and no communications satellites or satellite TV.

Lenses had existed for centuries, but until the 17th century, no one had thought of putting them together, one in front of another, creating what is today called a refracting telescope.

Hans Lippershey was credited with creating and disseminating the design for the first practical telescope. He applied to the States General of the Netherlands on October 2, 1608 for a patent that was subsequently denied. Lippershey’s device only magnified objects by three times. Almost immediately, telescopes were manufactured in large numbers and quickly spread all over Europe.

Galileo, the famous scientist, heard about the “Dutch perspective glass” in May 1609 while he was visiting Venice. Galileo wrote that he solved the problem of how to construct a telescope the first night after his return to Padua and he put together his first telescope the very next day. He then demonstrated his instrument to Leonardo Donato in Venice in the senate, who then set him for life in his lectureship at Padua.

Galileo quickly improved his telescope design, vastly increasing its magnifying power so that by 1610 he had an instrument that could magnify 33 times. With this instrument, he discovered four of the moons of Jupiter, saw the phases of Venus, sunspots on the sun, and mountains and craters on the moon. His discovery of the phases of Venus and the moons of Jupiter confirmed Copernicus’ theory that the sun was the center of the solar system instead of the Earth.

It was Galileo’s instrument that was first given the name “telescope” by an unidentified Greek poet and theologian present at a banquet held in 1611 by Prince Federico Cesi when Galileo was made a member of the Accademia dei Lincei, an Italian science academy in Rome. Thus, despite the fact that Lippershey is the actual inventor of the refracting telescope, it came to be called a Galilean Telescope.

Refracting telescopes like Galileo’s are not the most common astronomical instruments today. They suffered from several problems. Their lenses can only be made so large before they become too heavy to be held in a tube without deforming. Therefore, the largest refractor ever made is the one at the Yerkes Observatory; it is barely 40 inches across. Refracting telescopes also have trouble with aberration: that is, just as a prism splits light into colors, so a lens will tend to do that as well.

Therefore, refracting telescopes came to be generally replaced by a form that could be made considerably larger and cheaper without the color splitting problem: the reflecting telescope. Reflecting telescopes use a large curved mirror to do the same thing that a lens does. The ability of curved mirror to magnify and form images had been known for quite some time and in 1616 Niccolo Zucchi, an Italian Jesuit astronomer and physicist, produced the first telescope using a curved mirror instead of a lens. It was not the most practical instrument, since the observer had to look at the mirror and thus blocked part of the image with his head. Still, Zucchi was able to use it to discover the belts of Jupiter in 1630.

In 1672 Sir Isaac Newton devised a more practical form of reflecting telescope by adding a small flat diagonal mirror to reflect the light from the curved mirror up to a eyepiece mounted on the side of the telescope. This design for a reflecting telescope is still in use today and is called a Newtonian telescope.

A modification of Newton’s design was made by Laurent Cassegrain in 1672. He drilled a hole in the center of the curved mirror, and then placed a smaller mirror at the place where the curved mirror focused the light, directing that light back down and through the center hole to where the eyepiece was then affixed. These sorts of reflecting telescopes are now called Cassegrain telescopes. This is the most common design of all large astronomical instruments today (with modifications by several later scientists, such as Schmidt, Maksutov, Rictchey, and Chretein). Notable examples of Cassegrain telescopes include the famous 200 inch Hale reflector on Mt. Palomar in California, the giant Keck Telescopes in Hawaii (twin telescopes 394 inches in diameter), and the Hubble Space Telescope (94 inches in diameter) in orbit around the Earth.

One issue that ground based instruments suffered from since their invention was the distortion caused by the atmosphere on Earth: what makes stars seem to twinkle when you look at them at night. As a result, telescopes have been built on mountains where the weather and air are as stable and thin as possible, or in the case of Hubble, placed in space above the atmosphere. But in the 1990s a new technology, borrowed from the military, came into use known as adaptive optics. It works by measuring the distortions in the air with a laser and then compensating for that distortion by rapid changes of actuators applied to a deformable mirror or to a liquid crystal array filter. This allows ground based instruments to filter out the distortions caused by the air and allows them to see nearly as clearly as the Hubble Space Telescope—and in some instances better. However, the Hubble and its sister space scopes (such as the Spitzer) have advantages that can’t be overcome in any other way: they can see in wavelengths of light that are otherwise filtered out by the Earth’s atmosphere.

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