Cosmic Decoders: More About Telescopes
Topics:
Telescopes in General
Visible Light Telescopes
Radio Telescopes
Infra-red Telescopes
X-ray Telescopes
Gamma-ray Telescopes
Some Introductory Web Sites about Telescopes
A telescope is an instrument for collecting light from objects in space. The bigger the collector, the more light the telescope can gather. With more light, astronomers can see fainter and fainter objects.
Most of the objects out there are so far away that only a tiny bit of their light reaches the Earth (also see Learn More About Deep Space Objects and Cosmic Terms). Most stars, for example, are so distant that they cannot be seen by the human eye without a telescope. The stars we can see represent a tiny fraction of all the stars that are out there. So telescopes are essential for probing the universe in depth. In general, the farther out we want to look, the bigger the telescope we need.
Some telescopes collect visible light ˆ× the kinds of waves our eyes can see. Others collect invisible forms of light, such as radio waves or x-rays. The different types of telescopes are explained in a little more detail in the next sections.
A telescope can collect lots of light, but then some instrument has to detect that light ˆ× to receive and do something with the information in the light. For most of human history, the only detectors we had were human eyes, which, being attached to the human brain, were not always fully reliable. But then better detectors became available with the advent of film and electronic devices (as described below), and these are much more reliable and efficient than our eyes alone.
Film in a camera can make a permanent record of the light the telescope collects, and can also record that light for a much longer time than the human eye. This is called making a long exposure; astronomers will sometimes expose their film to the light of an object for hours ˆ× collecting more and more of the faint but precious information coded in the light.
Today, electronic detectors called “charge-coupled devices” (or CCD’s) are used to make very good digital recordings of the light from celestial objects. The word digital means that the amount of light from each part of the object is recorded as a number, not as a picture. By the way, the same kinds of CCD’s are used in modern video cameras to make good home recordings. CCD’s are much better ˆ× i.e., more efficient ˆ× at collecting light than film is. Film captures only about 1% of the light falling on it, while modern CCD’s can capture as much as 60 to 70% of the light that hits them. You can see why CCD’s are quickly replacing film photography for astronomers around the world. Detectors are also available for telescopes that collect invisible forms of light. In fact, during the twentieth century, scientists and engineers developed a host of clever devices for detecting and analyzing every possible kind of wave that the universe sends us.
In a visible light telescope, sometimes called an optical telescope, the light is collected by either a lens or a mirror. As light travels through a lens, it is bent (or refracted) to a focus. Therefore, telescopes that use lenses are called refractors. A mirror, on the other hand, will reflect the light to a focus off a shiny surface; telescopes with mirrors are thus called reflectors. Both lenses and mirrors are shaped to bring together all the light that hits their surfaces; and the more light that is collected, the brighter the image of the objects being viewed.
Visible light is only a tiny part of the huge range of waves the universe sends us. But stars like the Sun are especially good at giving off visible light, so visible light telescopes are extremely useful for studying the stars. This connection between our vision and starlight is no coincidence. Our eyes evolved to use the light that our star, the Sun, puts out. Thus our species is adapted to see the light of stars like the Sun. It’s as if looking at star light was built into our senses ˆ× no wonder everyone likes astronomy!
There are objects and processes in the universe that put out natural radio waves. If we were to translate these waves into sound, they would sound like the static that you hear between radio stations. Such cosmic radio waves can, for example, tell us about the raw material that lies between the stars, about sites of cosmic explosiveness, and about the remains of some types of dead or dying stars.
To gather radio waves from space, astronomers use giant metal antennas otherwise known as radio dishes (as most resemble a soup bowl of some sort). Radio waves are collected by these antennas and are brought to a focus, where a radio detector can then capture the waves. Later, the information in the waves is translated into a drawing (a “map”), or a graph that shows its various characteristics. (See for example, http://www.nrao.edu/imagegallery/php/level1.php).
Individual radio telescopes generally can’t show as much detail in cosmic objects as visible light telescopes do. To overcome this difficulty, astronomers frequently tie together a number of radio dishes into an array. The more widely spaced the telescopes in such an array, the more detail they can make out in the objects they observe. In the largest arrays, radio dishes are spread out over many miles, so that we can see the details of distant objects much more clearly than with any single radio telescope.
Objects that are cooler than stars don’t give off visible light, but they often give off infra-red waves (sometimes called heat waves). Planets, clouds of cosmic dust, and the chair in which you are sitting reading this web page all give off infra-red waves. One of the biggest problems with trying to detect faint infra-red waves from space is that everything on Earth (including everything around the astronomer, and the astronomer for that mater) is giving off infra-red waves. So infra-red telescopes need to be isolated and cooled to very low temperatures so their own infra-red waves don’t interfere with the waves coming from objects in space.
Since water vapor in our atmosphere gobbles up infra-red waves (not letting it through from space), infra-red astronomy is best done from above the clouds. Thus, modern infra-red telescopes are found in airplanes that can fly at high altitudes or on satellites in space. (For a nice tutorial on infra-red astronomy, see http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/index.html)
Very hot or very energetic objects in space can give off x-rays, the very same kind of waves your dentist uses to see how big your cavities are. X-rays from space, however, cannot penetrate the Earth’s atmosphere, and thus we must observe them from space. Since the dawn of the space age, several countries have launched x-ray telescopes on satellites that orbit the Earth. When such satellites make an observation, they record it in the form of numbers and send the information back to Earth using radio waves (which do get through our atmosphere).
On Earth, astronomers can assemble the information into an x-ray “picture” ˆ× a map of where the x-rays are intense and where they are weak. With modern x-ray telescopes, we are starting to put together a whole album of what the sky would look like if we had x-ray sensitive eyes. (See: http://chandra.harvard.edu)
The hottest or most energetic objects in space give off gamma-rays, the most energetic waves we know. Gamma-rays from space cannot penetrate the Earth’s atmosphere, and thus we must observe them from space. It is only in the last two decades that astronomers have realized that there ARE objects out there that give off gamma rays and thus only recently have we built and launched gamma-ray telescopes on satellites that orbit the Earth. When such satellites make an observation, they record it in the form of numbers and send the information back to Earth using radio waves (which can make it through our atmosphere).
Some Introductory Web Sites about Telescopes
How Telescopes Work:
http://science.howstuffworks.com/telescope.htm
The History of the Telescope (Visible Light):
http://es.rice.edu/ES/humsoc/Galileo/Things/telescope.html
It Takes More than One Kind of Telescope to See the Light:
http://science.nasa.gov/newhome/headlines/features/ast20apr99_1.htm
If you have questions about Cosmic Decoders, please contact us at astro {at} astrosociety.org.
