While no one knows for certain who invented the telescope, Galileo was the force behind its development. He built his first telescope in 1609 and later discovered the nature of the Milky Way, the satellites of Jupiter, the phases of Venus, the contours of the moon, the rotation of the sun, and sun spots.

An amateur seeking a first telescope would likely choose between reflecting and refracting scopes. Reflectors use mirrors and typically enable you to see more deeply into the sky than refractors, which use lenses and are best for magnifying the details of objects that are relatively close, such as the moon and planets. Experts are emphatic about one piece of advice: Visit a local astronomy club.

Besides visible light, large, professional telescopes also examine such different types of electromagnetic radiation, or “light,” as ultraviolet light, infrared, X rays, gamma rays, and radio waves.

At present, each of the twin Keck telescopes, with 10-meter-diameter primary mirrors, at Mauna Kea, Hawaii, qualifies as the world’s largest optical telescope. Should the four main 8-meter telescopes of the European Southern Observatory’s Very Large Telescope be used together, their reflectors would have more total area than the two Kecks. When completed in the year 2003, the Large Binocular Telescope on Mt. Graham in Arizona will be the largest telescope in the world on a single mounting-two 8.4-meter telescopes mounted like binoculars. For many observations, the diameter of the primary mirror is not as important as how efficient it is at concentrating starlight into images. Many telescopes that are in use or are just coming online in the 6.5- to 10-meter class are near each other in efficiency.

The Hubble Space Telescope runs on sunlight, making it the ultimate cordless power tool. In its 11 years of surveying the heavens, it has made 390,000 exposures and probed 15,700 celestial targets.

NASA’s Next-Generation Space Telescope, expected to replace Hubble, will be far more sensitive and able to see much farther. It should be able to see the first luminous objects that formed after the Big Bang.