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.
1. The primary purpose of any telescope is to produce an enormously magnified image of the field of view. HINT
2. A refracting telescope cannot form an image of its field of view. HINT
3. A Newtonian telescope has no secondary mirror. HINT
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. HINT
5. The term "seeing" is used to describe how faint an object can be detected by a telescope. HINT
6. The primary advantage to using the Hubble Space Telescope is the increased amount of "night" time available to it. HINT
7. One of the primary advantages of CCDs over photograph plates is their high efficiency in detecting light. HINT
8. The Hubble Space Telescope can observe objects in the optical, infrared, and ultraviolet parts of the spectrum. HINT
9. The Keck telescopes contain the largest single mirrors ever produced. HINT
10. Radio telescopes are large, in part to improve their angular resolution, which is poor because of the long wavelengths at which they observe. HINT
11. Radio telescopes are large, in part because the sources of radio radiation they observe are very faint. HINT
12. Radio telescopes have to have surfaces as smooth as those in optical telescope mirrors. HINT
13. Infrared astronomy must be done from space. HINT
14. Because the ozone layer absorbs ultraviolet light, astronomers must make observations in the ultraviolet from the highest mountain tops. HINT
15. Gamma-ray telescopes employ the same basic design as optical instruments. HINT
1. A telescope that uses a lens to focus light is called a _____ telescope. HINT
2. A telescope that uses a mirror to focus light is called a _____ telescope. HINT
3. All large modern telescopes are of the _____ type. HINT
4. The light-gathering power of a telescope is determined by the _____ of its mirror or lens. HINT
5. The angular resolution of a telescope is limited by the _____ of the telescope and the _____ of the radiation being observed. HINT
6. The angular resolution of ground-based optical telescopes is more seriously limited by Earth's _____ than by diffraction. HINT
7. Optical telescopes on Earth can see angular detail down to about _____ arc second. HINT
8. CCDs produce images in _____ form that can be easily transmitted, stored, and later processed by computers. HINT
9. Active optics and adaptive optics are both being used to improve the _____ of ground-based optical telescopes. HINT
10. All radio telescopes are of the _____ design. HINT
11. An _____ is two or more telescopes used in tandem to observe the same object, in order to improve angular resolution. HINT
12. ______, ______, and ______ astronomy can be done only from above Earth's atmosphere. HINT
13. An object with a temperature of 300 K would be best observed with an ______ telescope. HINT
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. HINT
15. Gamma-ray telescopes are unable to form ______ of their fields of view. HINT
1. Cite two reasons why astronomers are continually building larger and larger telescopes. HINT
2. What are three advantages of reflecting telescopes over refracting telescopes? HINT
3. How does Earth's atmosphere affect what is seen by an optical telescope? HINT
4. What advantages does the Hubble Space Telescope have over ground-based telescopes? List some disadvantages. HINT
5. What are the advantages of a CCD over a photographic plate? HINT
6. What is image processing? HINT
7. Describe some ways in which optical astronomers can compensate for the blurring effects of Earth's atmosphere. HINT
8. Why do radio telescopes have to be very large? HINT
9. What kind of astronomical objects can we best study with radio techniques? HINT
10. What is interferometry, and what problem in radio astronomy does it address? HINT
11. Compare the highest resolution attainable with optical telescopes with the highest resolution attainable with radio telescopes (including interferometers). HINT
12. What special conditions are required to conduct observations in the infrared? HINT
13. Compared with optical astronomy, what new problems arise when we wish to make observations in the high-energy domain? HINT
14. What is the main advantage of studying objects at different wavelengths of radiation? HINT
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? HINT
1. A certain telescope has a 10'10' field of view that is recorded using a CCD chip having 1024 
1024 pixels. What angle on the sky corresponds to 1 pixel? What would be the diameter of a typical seeing disk (2" radius), in pixels? HINT
2. Estimate the fraction of incoming starlight obscured by the observer and observing cage at the prime focus of the Hale 5-m telescope (see Figure 5.8). HINT
3. A 2-m telescope can collect a given amount of light in 1 hour. Under the same observing conditions, how much time would be required for a 6-m telescope to perform the same task? A 12-m telescope? HINT
4. A certain space-based telescope can achieve (diffraction-limited) angular resolution of 0.05' for red light (of wavelength 700 nm). What would its resolution be (a) in the infrared, at 3.5 µm, and (b) in the ultraviolet, at 140 nm? HINT
5. The photographic equipment on a telescope is replaced by a CCD. If the photographic plate records 5 percent of the light reaching it, while the CCD records 75 percent, how much time will the new system take to collect as much information as the old detector recorded in a 1-hour exposure? HINT
6. The Andromeda Galaxy lies about 2.9 million light years or 900 kpc away. To what distances do the angular resolutions of the Hale telescope (1"), HST (0.05"), and a radio interferometer (0.001") correspond to at that distance? HINT
7. Based on collecting areas, how much more sensitive would you expect the Arecibo telescope (Figure 5.21) to be, compared with the 43-m Green Bank instrument (Figure 5.20)? HINT
8. What would be the equivalent single-mirror diameter of a telescope constructed from two separate 10-m mirrors? Four separate 8-m mirrors? HINT
9. Estimate the angular resolutions of (a) a radio interferometer with a 5000-km baseline, operating at a frequency of 5 GHz, and (b) an infrared interferometer with a baseline of 50 m, operating at a wavelength of 1 µm. HINT
10. An instrument aboard the Gamma Ray Observatory detects a burst of energy coming from some distant source. Simultaneously, an X-ray telescope with an effective diameter of 0.5 m observes the same event in the 2 keV energy range (see More Precisely 4-2 for a definition of the electron volt, or eV). Both detectors record the location of the event on the sky, to within the accuracy of their respective measurements. You are about to make a follow-up observation using a large optical telescope with a 5'field of view. Which position—the X-ray or the gamma-ray measurement—should you use, and why? HINT
1. Here's how to take some easy pictures of the night sky. You will need a location with a clear, dark sky, a 35mm camera with a standard 50-mm lens, tripod, and cable release, a watch with a seconds display visible in the dark, and a roll of high-speed color film.
Set your camera to the "bulb" setting for the exposure and attach the cable release so you can take a long exposure. Set the focus on infinity. Point the camera to a favored constellation, seen through your viewfinder, and take a 20- to 30-second exposure. Don't touch any part of the camera or hold on to the cable release during the exposure; minimize all vibration. Keep a log of your shots. When finished, have the film developed in the standard way.
2. For some variations, vary your exposure times, use different films, take hours-long exposures for star trails, use different lenses such as wide-angle or telephoto, place the camera piggyback on a telescope that is tracking, and take exposures that are a few minutes long. Experiment and have fun!
