The Story of the Telescope — from a Single Lens to Space Observatories
From the world's first telescope to Kepler's refractor, Newton's reflector, and on to Hubble and James Webb lofted into space. Let's follow, with diagrams, the path humankind has walked to see the universe ever farther and ever deeper.
The world's first telescope
The telescope was not invented to look at the stars. In 1608 the Dutch spectacle-maker Hans Lippershey discovered that holding two lenses one behind the other made distant things appear near, and the patent he applied for is the first telescope on record. At first it was regarded mainly as an "instrument for seeing far away," used to watch distant ships or the movements of an enemy.
The following year, in 1609, the Italian Galileo Galilei heard of it, built his own greatly improved telescope, and became the first human to turn one on the night sky. Its magnification was barely more than twenty times, yet that little tube revealed the mountains of the Moon and the moons of Jupiter, changing forever the way we see the universe.
The refractor — the Galilean and Keplerian designs
The principle of the telescope is surprisingly simple: to gather as much light as possible from a faint object and show it brighter and larger than the eye can. In a refracting telescope, a convex "objective lens" focuses starlight, and an "eyepiece" magnifies that image for viewing.
The early form Galileo used (the Galilean design) used a concave lens for the eyepiece, giving an upright image but a very narrow field of view. In 1611 Johannes Kepler devised the "Keplerian telescope," which made the eyepiece a convex lens too; the image appears upside down, but the field of view is wide and it is easy to reach high magnification — which is why it became the basic principle behind the eyepiece end of today's astronomical telescopes.
The reflector — Newton's mirror
Lenses grow heavy as they get larger, and because each color of light focuses at a slightly different point, they smear images into a rainbow — an effect called "chromatic aberration." In 1668 Isaac Newton solved this by building the first reflecting telescope, which gathers light with a concave "primary mirror" instead of a lens. Mirrors produce no color smearing and, being supported from behind, are easy to make large — so almost all of today's great observatory telescopes are reflectors.
The single most important figure governing a telescope's power is not magnification but "aperture" (the diameter of the lens or mirror). The larger the aperture, the more light it gathers, letting it see fainter and more distant objects ever more clearly. And so today, atop the mountain peaks of Hawaii and Chile stand giant telescopes 8 to 10 meters across, while colossal ones 30 to 39 meters in diameter are built from dozens of hexagonal mirrors joined together like a honeycomb.
Seeing by invisible light
Our eyes can see only a very narrow band of light (visible light), but the universe pours out all kinds of light together — radio waves, infrared, ultraviolet, X-rays, and gamma rays. So modern astronomy uses special telescopes to capture each of them. The "radio telescope," which gathers radio waves with a giant dish, took the first-ever image of a black hole's shadow in 2019, while infrared telescopes peer straight through veils of dust to the birthplaces of stars.
Beyond the atmosphere — the Hubble Space Telescope
A ground-based telescope's greatest enemy is the Earth's atmosphere. When the air shimmers, starlight wavers and the image blurs. So humankind lifted its telescopes clear of the atmosphere altogether. In 1990 the 2.4-meter "Hubble Space Telescope" rose into orbit about 540 km above the Earth. At first its mirror had been polished slightly wrong and its images were fuzzy, but after astronauts flew up in 1993 and fitted it with corrective optics, it has sent back flawlessly crisp pictures for more than thirty years.
Hubble measured the expansion rate of the universe with precision and, in a single image called the "Hubble Deep Field," captured thousands of galaxies, transforming astronomy wholesale. In 2021 its successor, the "James Webb Space Telescope," rose into space with its 6.5-meter golden mirror folded up, and in infrared it now captures even the faint 13.8-billion-year-old light of the universe's very first galaxies. The path that began with two lenses has at last reached all the way to peering into the dawn of the cosmos.