(Background) This semi-transparent illustration shows some of the main features of the Hubble Space Telescope. The large blue disk at center of the spacecraft is the primary mirror, and the red gadgets to its rear are the sensors that guide the pointing of the telescope. The open aperture door is at upper left. The huge solar panels are shown in yellow-checkered blue at left and partly obscured at right. Looking inside the aft bay of the vehicle, we can see key components of each of the science instruments—the spectrometers are shown in copper and blue (in the foreground), the cameras in pink, green and lavender (mostly in the background).

(Insets A, B, C, and D) The four small insets are computer-rendered views of Hubble in orbit. Despite having the size of a city bus, Hubble is designed to move in space with the grace of a prima ballerina. All these illustrations are taken from video animations made by the astronomy artist and animator Dana Berry.

LEARNING GOALS

Studying this chapter will enable you to:

	Sketch and describe the basic designs of the major types of optical telescopes used by astronomers.
	Explain why very large telescopes are needed for most astronomical study, and specify the particular advantages of reflecting telescopes for astronomical use.
	Describe how Earth's atmosphere affects astronomical observations, and discuss some of the current efforts to improve ground-based astronomy.
	Discuss the advantages and disadvantages of radio astronomy compared to optical observations.
	Explain how interferometry can enhance the usefulness of radio observations.
	List the other types of nonvisible radiation currently being exploited for astronomical observation, and summarize the advantages, limitations, and chief uses of each.
	Tell why it is important to make astronomical observations in different regions of the electromagnetic spectrum.

At its heart, astronomy is an observational science. More often than not, observations of cosmic phenomena precede any clear theoretical understanding of their nature. As a result, our detecting instruments—our telescopes—have evolved to observe as broad a range of wavelengths as possible. Until the middle of the twentieth century, telescopes were limited to visible light. Since then, technological advances have expanded our view of the universe to all regions of the electromagnetic spectrum. Some telescopes are sited on Earth, whereas others must be placed in space, and design considerations vary widely from one part of the spectrum to another. Whatever the details of their construction, however, telescopes are devices whose basic purpose is to collect electromagnetic radiation and deliver it to a detector for detailed study.