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before hubble
Galileo's telescope Telescopes: Our eyes on the universe

by Kelly LaMarco
Adapted from the print edition of Exploratorium Magazine

Credit for inventing the telescope usually goes to Hans Lippershey, a Dutch optician. Lippershey, who revealed his device for seeing things at a distance in 1608, probably intended that it would be used primarily by sailors. Word of the instrument traveled to the famous scientist Galileo Galilei in Italy, who decided to make his own. One night in 1609, Galileo turned his telescope to the heavensand changed astronomy forever. Telescopes, even very early ones, revealed information about the celestial entities that no one had previously imagined. Since then, astronomers have used telescopes to study everything from our moon to comets, star clusters, and distant galaxies. And with a telescope that's currently in the planning stages, astronomers hope someday to see to the very beginning of the universe.

Most telescopes work in essentially the same way, producing an image of a distant object inside a tube; we can then look at the image through an eyepiece. A telescope helps us to perceive faraway elements, not primarily through magnification, as commonly believed, but through its ability to collect light. The amount of light collected by a telescope determines the level of detail we can discern and is related to the size of its light-collecting component, commonly either a lens or a mirror (referred to as the telescope's "primary optics"). The larger the light-collecting component, the greater the telescope's light-gathering ability and, therefore, its "resolving power." Resolving power is the ability of the telescope to show two adjacent objects, such as "double" stars or a planet and its moon, as separate, distinct images.

Newton's Reflecting Telescope
Because he couldn't overcome the problem of chromatic aberration, Sir Isaac Newton decided to use a mirror in a telescope in place of a lens. The result was the first reflecting telescope, which he invented in 1672.

The eyepiece, which consists of two or more lenses mounted in a cylindrical metal barrel, acts like a magnifying glass held up to the image produced by the primary optics. The magnifying power of a telescope (for example, 20X or 20-power) can be altered by changing the eyepiece.

Modern optical telescopes fall into two general categories: refracting telescopes, such as the land-based 91-centimeter (36-inch) refractor at the Lick Observatory near San Jose, California; and reflecting telescopes, including the Hubble Space Telescope.

The Hubble Space Telescope (HST) is a large reflecting telescope that was designed by NASA to be a space-based observatory with a life expectancy of about fifteen years. Set into orbit in 1990, its position high above the earth's atmosphere means that it doesn't have to contend with particles in the air and air movement that distort the view from ground-based telescopes. The Hubble has brought us startling images of the moons of Jupiter, the rings of Saturn, and nebulae alive with the light of newly formed stars.

An important feature of the HST is its sensitivity to ultraviolet light (UV). UV is absorbed by molecules in the earth's atmosphere. Because the HST orbits above the atmosphere, it's able to make observations in ultraviolet light. The UV portion of the spectrum provides information about the properties of celestial bodies, such as their chemical compositions.

The HST's space mission will end in a decade or so, but NASA has not been idle. The Next Generation Space Telescope (NGST), part of NASA's Origins Program, will be the successor to the HST. The ultimate goal of the NGST is to gain an understanding of how our universe evolved after the Big Bang. Because the light that comes to us from the very distant first stars and galaxies in the universe has a frequency lower than the frequency range of visible light, their study requires observations to be made in the infrared portion of the spectrum. The NGST is designed to have these capabilities, unlike the HST, which operates from the ultraviolet to the near infrared.

Hubble Space Telescope
Hubble is a reflecting telescope with a primary mirror that is 2.4 meters (94.5 inches) wide, smaller than the largest observatory telescopes. But because it doesn't have to contend with atmospheric distortion, the resolution of Hubble is about ten times greater than high-quality ground-based telescopes.

But you don't have to be part of a NASA mission to study the stars. Astronomy is one area of science where amateurs regularly contribute to the body of knowledge. Amateur astronomers all over the world have their telescopes poised to discover new comets and to patrol distant galaxies for supernovas. Amateurs also watch selected "variable" starsstars whose brightness varies in a cyclical mannerand take down data such as how bright the star is at different times. They then send the data to professional astronomers, who use the information in their research. Another astronomical activity that is gaining popularity is watching the sky for space objects such as comets and asteroids. The popular press has made the public aware of such potential visitors, and increasing numbers of people are on the lookout for so-called "near earth orbit" objects (NEOs).

If you simply want to use a telescope to enjoy the beauty and mystery of the heavens, you can view the four largest moons of Jupiter and the rings of Saturn through most commercially available telescopes. With respect to constellations, the seasonal skies provide a changing light show. During the winter in the Northern Hemisphere, Orion is a prominent constellation, containing the red star Betelgeuse and the spectacular greenish clouds of the Orion nebula. The Ursa Major or "Great Bear" constellation, whose seven brightest stars form the Big Dipper, is prominent in the Northern Hemisphere's summer sky. But there is no reason to limit your viewing to the constellations, however wondrous they are. Also starring in each evening's performance are planets, nebulae, and our moon. Enjoy the show!





© 2001 Exploratorium