Most of the universe is invisible to us because we only see the visible light portion of the
            electromagnetic spectrum. When most people think of telescopes they think of visible light,
            or optical, telescopes.
 

When the first optical telescope appeared in the              1570s, the design was simple - one concave and one             convex lens fitted inside a tube. The tube acted as a             receiver, or 'light bucket'. The lenses bent, or             refracted, the light as it passed through the glass and             thus made the scene appear 3 to 4 times larger.

              Galileo improved upon the design and by 1609 had developed a 20-power refracting
           telescope. Galileo made the telescope famous when he discovered the valleys and
           mountains of the moon and spotted four of Jupiter's satellites.
 

Left: Galileo Galilei (1564-1642), Italian astronomer, mathematician and physicist.The glass lenses in the Galileo telescope weren't very clear,  however - they were full of little bubbles and had a greenish tinge due to  the iron content of the glass. Also, the shape of the glass lenses gave the  field of view very fuzzy edges.

            The magnification of Galileo's telescope could only be improved by focusing the light farther
            behind the primary lens, which resulted in longer and longer telescopes. But once telescopes

reached 140 feet in length they became almost             useless for observation. It was impossible to keep             the lenses properly aligned at such long lengths.             Longer telescopes also required larger lenses, and             after a lens reached 1 meter (3.28 ft.) in diameter it             would deform, sagging under its own weight.             Right: Johannes Hevelius' 150-ft. telescope (Machina             Coelestis, 1673). Reprinted with permission by the             Royal Astronomical Society, London.             Isaac Newton invented the first reflecting telescope             in 1671. By using a curved mirror to reflect and

              focus the light inside the tube, he was able to reduce the length of the telescope dramatically.
           The reflecting telescope solved another problem inherent in the refracting telescope:
           chromatic aberration.

            In 1672, Newton described how white light is actually a mixture of colored light. Each color
 

has its own degree of refraction, so curved lenses split white light into  the colors of the spectrum. This chromatic aberration caused central  images in refracting telescopes to be surrounded by rings of different  colors.  Planets seen through a refracting telescope would appear to be  encircled by a rainbow. Left: Sir Isaac Newton (1642-1727), English mathematician and physicist. By 1730, Newton's reflecting telescope had caught on with the scientific

            community. Even today, large optical telescopes are based upon Newton's basic design. Yet

another bonus of Newton's reflecting             telescope is that it can also be used to study             ultraviolet and infrared light. The Hubble             Space Telescope, famous for its stunning             optical images of the universe, also works in             the ultraviolet and infrared parts of the             spectrum.

            But it wasn't until the 1930s that astronomers even began looking for other parts of the
            electromagnetic spectrum. Karl Jansky inadvertently discovered galactic emissions of radio
            waves in 1933. Working at Bell Telephone Laboratories, Jansky was trying to find what
           caused short-wave radio interference in Trans-Atlantic communications. By building a
           rotating radio telescope to look at the horizon, he eventually discovered that most of the
           static resulted from engine ignition noise and distant lightning storms. But Jansky
 

also discovered that some radio noise was coming from thecenter  of the Milky Way Galaxy. Left: The "Jansky Antenna" doesn't look much like modern aerials, i.e., TV antennas or satellite dishes, because it was designed to receive shortwave signals coming over the horizon.

            Like optical telescopes, radio telescopes have reflectors and receivers. Most radio telescopes
            need to be large in order to accommodate radio's longer wavelengths and lower energies.
            Resolution is also a factor: low-frequency radio waves would be unfocused and fuzzy in

smaller telescopes. Radio telescopes also need to                be large in order to overcome the radio noise, or             "snow," that naturally occurs in radio receivers.             We generate a large amount of noise on Earth             as well, so smaller telescopes would lose some             astronomical radio signals amid our daily             production of rock music, television broadcasts             and cellular phone calls. An example of a             modern radio telescope is The Very Large             Array in New Mexico (right), composed of 27             antennas electronically combined to give the             resolution of an antenna 36 kilometers (22             miles) across.

             Radio and optical telescopes can be used on Earth, but some resolution is lost due to Earth's
            atmosphere. By viewing from the other side of the sky, the Hubble Space Telescope allows
            astronomers to see the universe without the distortion and filtering that occurs as light passes
            through the Earth's atmosphere.

            Infrared and ultraviolet light are affected more dramatically by the Earth's atmosphere. Their
            telescopes must therefore always be positioned high above the ground or in space. Infrared
 

telescopes are placed on mountaintops, far above the low-lying water vapor that interferes with infrared light. Left: The NASA Infrared Telescope Facility 3.0 meter telescope at the summit of Mauna Kea, Hawaii.  Photo courtesy of Ernie Mastroianni.