show #1209

David joins astronaut Jeff Hoffman for a close look at space. Segment length: 7:47


Insights & Connections



Main activity

Try this


Is there anybody out there? Just as ancient explorers ventured out onto uncharted seas, astronomers have devised clever ways to explore space. Up until the early 1600s, the only tools people had for observing the night sky were their eyes and navigational instruments like sextants, astrolabes, and cross staffs. A major breakthrough came in 1609 when Galileo borrowed an idea from a Dutch lens maker and created the astronomical telescope.

With it, Galileo determined that Venus went through phases like our moon, that Jupiter had moons orbiting it, and that Saturn had protuberances of some kind. Based on these discoveries, he challenged Aristotle's Earth-centered model of the universe and the science of astronomy took a quantum leap forward.

Over the next hundred years, telescope-making became a well-established art. Late in the 1600s, Isaac Newton discovered that refracting lenses weren't the only way to bring distant objects close. By placing a concave mirror at the base of a hollow tube, he could concentrate more light with much less distortion. Using these new reflecting telescopes, astronomers like Caroline Herschel, George Hale, and Edwin Hubble pushed the limits of the visible universe even farther.

By the early twentieth century, reflecting telescopes grew to mammoth proportions. Because Earth is blanketed by a layer of air over 100 kilometers (62 miles) thick, ground-based telescopes must "see" through a soup of air currents, water vapor, and dust in the atmosphere. This distorts the light passing through the atmosphere and ultimately the image collected by the telescope.

The only way to get rid of distortion caused by the atmosphere is to place a telescope in orbit above it. In 1990, NASA used the space shuttle to place the Hubble Space Telescope (HST) in orbit around Earth at an altitude of about 613 kilometers (380 miles). HST is actually a combination of both optical and spectrographic instruments which capture light that is collected by HST's primary mirror. These instruments use computers to digitize images and send them back to Earth via radio transmissions. The digitized images are reassembled into pictures by computers on the ground. The primary mirror of HST is only 2.4 meters (8 feet) in diameter, which is small compared to many ground- based scopes. But because it's above the atmosphere, it can capture light from a volume of space almost 300 times greater than Earth-bound scopes.

Unfortunately, soon after its launch, project scientists found that the HST primary mirror had a slight imperfection which impaired its focusing power. In optical terms, this imperfection is called spherical aberration. Shuttle astronauts repaired the problem and gave HST better focus. By having HST in orbit above Earth's atmosphere, astronomers have now seen further than anyone has seen before.


concave a curved surface that is bent inward

convex a curved surface that is bent outward

focus the point at which all the individual light rays are concentrated

focal length the distance between the mirror and the focal point

lens a device used for bending and concentrating light

mirror a device used for reflecting light

reflecting telescope a telescope that uses mirrors to concentrate light from a distant source

refracting telescope a telescope that uses a lens to concentrate light from a distant source

spherical aberration an imperfection in a lens that causes light to be focused at more than one point, distorting the image


Additional sources of information

Lorain County Joint Vocational School
15181 Route 58 S.
Oberlin, OH  44074
(216) 774-1051, ext. 293 or 294
(catalog of A/V material available by request on school letterhead)

NASA Spacelink Attn: Spacelink Administrator Mail Code CA 21 NASA Marshall Space Flight Center Huntsville, AL 35812 (Computer information service supplies news about current NASA programs, activities, lesson plans, etc.)

Edmund Scientific 101 E. Gloucester Pike Barrington, NJ 08071 (609) 573-6250 (catalog for optics)


Have you ever wondered why the side mirror on your car says "objects in mirror are closer than they appear" or why your reflection always looks strange in fun house mirrors? This activity gives you the opportunity to experiment with several different types of mirrors to shed light on how they focus.


  1. Lay the flashlight flat on a table on top of a piece of white paper. Make the room as dark as possible and turn on the light so that the beam spreads out across the sheet of paper. Take the comb and place it in front of the light so that the beam shines through the teeth. You should see a series of light rays spreading out from behind the comb.
  2. Take the flat mirror and hold it up on edge so that it is facing the light rays coming from the flashlight. Use your pencil to trace the pattern of light rays as they strike and reflect off the flat mirror.
  3. After tracing the pattern, label the page "flat" and put it off to the side. Put a new sheet of paper under the light and place the spoon in front of the light rays so that the back of the spoon is facing the flashlight. Trace this reflection pattern on the second sheet and label it "convex." (Note: To be able to draw the light rays reflected from the spoon, you must place the paper and spoon at the edge of the table so that the rays are reflected from the center--not the edge--of the spoon.)
  4. Place the third piece of paper under the light and this time, have the light rays shine into the bowl of the spoon. Trace this pattern on the paper and label it "concave." Turn on the lights and compare the three reflection patterns.


  1. Compare the convex pattern with the flat pattern. How are they different? How might this explain why convex mirrors spread out the light from an object?
  2. Compare the concave pattern with the flat pattern. How does this pattern explain why concave mirrors concentrate the light from an object? Can you identify the focal point?
  3. Based on your experiment, why do you think wider-diameter concave mirrors are used to gather light from very distant astronomical objects?


Organize a star party for you and your friends. It's easy to do! All you need is a current star map available monthly in magazines like Sky and Telescope, Astronomy, or Natural History and a relatively dark place to view the night sky. You may want to contact a local amateur astronomy group to find out if they have regularly planned events. Or start your own club at school!


Try your hand at building a simple reflecting telescope. Simple, inexpensive plans are available in various science project books found in most libraries. Many basic parts can be obtained from local hardware stores or from a source like Edmund Scientific (see resources).


Contact a local observatory and see if they have any public viewing nights. You'd be surprised how many planetariums, colleges, and universities have facilities that are open to the public at little or no charge. If you have never seen the rings of Saturn or the Horsehead Nebula, it's well worth the trip.


Most people can see how dust, pollution, or even lights can interfere with viewing objects in space, but how can clear air obstruct a view? To see how simple heating and cooling of the air can cause you to see ripples through a telescope, try observing objects on the far side of a radiator when the radiator is hot. Another option would be to set up a hot plate on a table, turn it on high, and look over it at a small piece of aluminum foil taped to the wall behind it. If you're lucky, you should see the foil twinkle just like a distant star at night.
Newton's Apple is a production of KTCA Twin Cities Public Television. Made possible by a grant from 3M. Educational materials developed with the National Science Teachers Association.