What is the name for the tools astronomers use that collect as much light as possible from an astronomical source and focuses it down to a narrow beam to form an image?

TELESCOPE SUMMARY

A telescope is a device designed to collect as much light as possible from some distant source and deliver it to a detector for detailed study. Reflecting telescopes use a mirror to concentrate and focus the light. Refracting telescopes use a lens; refraction is the bending of light as it passes from one medium to another. The prime focus of the telescope is the point where the incoming beam is focused and where analysis instruments may be placed. The Newtonian and Cassegrain telescope designs employ secondary mirrors to avoid placing heavy equipment at the prime focus.

The lenses used in refracting telescopes suffer from a number of problems, among them chromatic aberration, the tendency of lenses to focus different colors to different prime foci. These problems become more difficult to correct the larger the lens is, with the result that all astronomical telescopes larger than about 1 m in diameter use mirrors in their design.

The light collected by a telescope may be processed in a number of ways. It can be made to form an image, a photometer may be used to make detailed measurements of the energy received, or a spectrometer may study its spectrum.

The light-gathering power of a telescope depends on its collecting area, which is proportional to the square of the mirror diameter. To study the faintest sources of radiation, astronomers must use large telescopes.

An important aspect of a telescope is its angular resolution, the ability to distinguish between light sources lying close together on the sky. One limitation on the resolution of a telescope is diffraction, which makes it impossible to focus a beam perfectly. The amount of diffraction is proportional to the wavelength of the radiation under study and inversely proportional to the size of the mirror. Thus, at any given wavelength, larger telescopes suffer least from the effects of diffraction.

The resolution of most ground-based optical telescopes is actually limited by seeing—the blurring effect of Earth's turbulent atmosphere, which smears the pointlike images of stars out into seeing disks a few arc seconds in diameter. Radio and space-based telescopes do not suffer from atmospheric effects, so their resolution is determined by the effects of diffraction.

Most modern telescopes now use charge-coupled devices, or CCDs, instead of photographic plates to collect their data. The field of view is divided into an array of millions of pixels that accumulate an electric charge when light strikes them. CCDs are many times more sensitive than photographic plates, and the resultant data are easily saved directly on disk or tape for later image processing.

Using active optics, in which a telescope's environment and focus are carefully monitored and controlled, and adaptive optics, in which the blurring effects of atmospheric turbulence are corrected for in real time, it may soon be possible to achieve diffraction-limited resolution in ground-based optical instruments.

Radio telescopes are conceptually similar in construction to optical reflectors. However, radio telescopes are generally much larger than optical instruments, for two reasons. First, the amount of radio radiation reaching Earth from space is tiny compared with optical wavelengths, so a large collecting area is essential. Second, the long wavelengths of radio waves mean that diffraction severely limits the resolution unless large instruments are used.

In order to increase the effective area of a telescope, and hence improve its resolution, several separate instruments may be combined into a device called an interferometer. Using interferometry, radio telescopes can produce images much sharper than those from the best optical equipment. Infrared interferometers are under construction, and optical interferometric systems are under active development.

Infrared telescopes and ultraviolet telescopes are similar in their basic design to optical systems. Infrared studies in some parts of the infrared range can be carried out using large ground-based systems. Ultraviolet astronomy must be carried out from space.

High-energy telescopes study the X- and gamma-ray regions of the electromagnetic spectrum. X-ray telescopes can form images of their field of view, although the mirror design is more complex than for lower-energy instruments. Gamma-ray telescopes simply point in a certain direction and count photons received. Because the atmosphere is opaque at these short wavelengths, both types of telescopes must be placed in space.

Radio and other nonoptical telescopes are essential to studies of the universe because they allow astronomers to probe regions of space that are completely opaque to visible light and to study the many objects that emit little or no optical radiation at all.

SELF-TEST: TRUE OR FALSE?

1. The primary purpose of any telescope is to produce an enormously magnified image of the field of view.

2. A refracting telescope cannot form an image of its field of view.

3. A Newtonian telescope has no secondary mirror.

4. A Cassegrain telescope has a hole in the middle of the primary mirror to allow light reflected from its secondary mirror to reach a focus behind the primary mirror.

5. The term "seeing" is used to describe how faint an object can be detected by a telescope.

6. The primary advantage to using the Hubble Space Telescope is the increased amount of "night" time available to it.

7. One of the primary advantages of CCDs over photograph plates is their high efficiency in detecting light.

8. The Hubble Space Telescope can observe objects in the optical, infrared, and ultraviolet parts of the spectrum.

9. The Keck telescopes contain the largest single mirrors ever produced.

10. Radio telescopes are large, in part to improve their angular resolution, which is poor because of the long wavelengths at which they observe.

11. Radio telescopes are large, in part because the sources of radio radiation they observe are very faint.

12. Radio telescopes have to have surfaces as smooth as those in optical telescope mirrors.

13. Infrared astronomy must be done from space.

14. Because the ozone layer absorbs ultraviolet light, astronomers must make observations in the ultraviolet from the highest mountain tops.

15. Gamma-ray telescopes employ the same basic design as optical instruments.

SELF-TEST: FILL IN THE BLANK

1. A telescope that uses a lens to focus light is called a _____ telescope.

2. A telescope that uses a mirror to focus light is called a _____ telescope.

3. All large modern telescopes are of the _____ type.

4. The light-gathering power of a telescope is determined by the _____ of its mirror or lens.  

5. The angular resolution of a telescope is limited by the _____ of the telescope and the _____ of the radiation being observed.

6. The angular resolution of ground-based optical telescopes is more seriously limited by Earth's _____ than by diffraction.

7. Optical telescopes on Earth can see angular detail down to about _____ arc second.

8. CCDs produce images in _____ form that can be easily transmitted, stored, and later processed by computers.

9. Active optics and adaptive optics are both being used to improve the _____ of ground-based optical telescopes.

10. All radio telescopes are of the _____ design.

11. An _____ is two or more telescopes used in tandem to observe the same object, in order to improve angular resolution.

12. ______, ______, and ______ astronomy can be done only from above Earth's atmosphere.

13. An object with a temperature of 300 K would be best observed with an ______ telescope.

14. The mirrors in X-ray telescopes are different in design from those in optical instruments because X-rays tend to be ______, rather than reflected, by solid surfaces.

15. Gamma-ray telescopes are unable to form ______ of their fields of view.

REVIEW AND DISCUSSION

1. Cite two reasons why astronomers are continually building larger and larger telescopes.

2. What are three advantages of reflecting telescopes over refracting telescopes?

3. How does Earth's atmosphere affect what is seen by an optical telescope?

4. What advantages does the Hubble Space Telescope have over ground-based telescopes? List some disadvantages.

5. What are the advantages of a CCD over a photographic plate?

6. What is image processing?

7. Describe some ways in which optical astronomers can compensate for the blurring effects of Earth's atmosphere.

8. Why do radio telescopes have to be very large?

9. What kind of astronomical objects can we best study with radio techniques?

10. What is interferometry, and what problem in radio astronomy does it address?

11. Compare the highest resolution attainable with optical telescopes with the highest resolution attainable with radio telescopes (including interferometers).

12. What special conditions are required to conduct observations in the infrared?

13. Compared with optical astronomy, what new problems arise when we wish to make observations in the high-energy domain?

14. What is the main advantage of studying objects at different wavelengths of radiation?

15. Our eyes can see light with an angular resolution of 1'. Suppose our eyes detected only infrared radiation, with 1° angular resolution. Would we be able to make our way around on Earth's surface? To read? To sculpt? To create technology?