- Why is an orbiting telescope better than one on the ground?
- What was wrong with the Hubble Space Telescope and how did the astronauts
David joins astronaut Jeff Hoffman for a close look at space.
Segment length: 7:47
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
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.
- Many critics argue that the money for government-funded space projects
could be better spent here on Earth. Do the spin-offs and scientific knowledge gained from space
programs justify their expense, or are they a waste of resources?
- Do you think there is life on other planets? If so, how might we go about
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
- Bunge, R. (1993, Aug) Big scopes: Dawn of a new era. Astronomy, pp. 48-53.
- Bruning, D. (1994, Apr) Hubble better than new. Astronomy, pp. 44-49.
- Bruning, D. (1994, Jan) Fixing Hubble. Astronomy, pp. 36-39.
- Chaisson, E. (1994) The Hubble wars. New York: Harper Collins.
- Davidson, G. (1993, Feb) How we'll fix Hubble. Astronomy, pp. 42-49.
- Macauley, D. (1988) The way things work. Boston: Houghton Mifflin.
- Smith, B. (1994, Jan) New eyes on the universe. National Geographic, pp.
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)
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.)
101 E. Gloucester Pike
Barrington, NJ 08071
(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.
- small, flat mirror
- three 8 1/2" x 11" sheets of plain white paper
- darkened room
- large, shiny spoon
- 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
- 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.
- 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.)
- 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
- 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?
- 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
- 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
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