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Chapter 7:
Optics of the Atmosphere

7.1 Introduction

The performance of ground-based telescopes is affected by the atmosphere in two ways: (1) absorption in the atmosphere limits the wavelength regions available for ground-based astronomy to several transparent or partially transparent windows; and (2) atmospheric turbulence limits the angular resolution of telescopes that are imaging relatively faint objects for long periods of time.

The earth's atmosphere is transparent across the windows from 350 to 2400 nm and in the IR from 3 to 5.1 μm as well as from 8 to 12 μm. Atmospheric absorption between these windows is caused primarily by molecules of CO, CO2 and H2O (water vapor). As mentioned, atmospheric turbulence limits angular resolution. Faint objects require long exposures, and the dynamic, changing atmosphere limits angular resolution to approximately 1 arcsec during long exposures. Short exposures freeze the atmospheric turbulence to give images of bright stars a speckled pattern. This pattern forms the basis of speckle interferometry and is discussed further below and in Chapter 9.

Many investigators have analyzed the propagation of an optical wavefront through the earth's atmosphere. In this chapter we briefly review theoretical developments and mention methods that enable high-angular-resolution imaging through atmospheric turbulence. These methods are discussed in detail in later chapters.

Astronomers compensate for atmospheric turbulence and obtain images at the maximum resolution of their telescopes using several methods. One is called adaptive optics. The second is called speckle interferometry. The third is called pupil wavefront folding interferometry. Each of these methods is discussed in a later chapter: astronomical speckle interferometry is discussed in Section 10.12.1, pupil wavefront folding interferometry (coherence interferometry) is discussed in Section 10.13, and adaptive optics is discussed in Chapter 11.

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