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1 Introduction‣ Essential Radio Astronomy

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Radio astronomy is the study of natural radio emission from celestial sources. The range of radio frequencies or wavelengths is loosely defined by atmospheric opacity and by quantum noise in coherent amplifiers. Together they place the boundary between radio and far-infrared astronomy at frequency ν∼1 THz (1 THz ≡1012 Hz) or wavelength λ=c/ν∼0.3 mm, where c≈3×1010⁢cm⁢s-1 is the vacuum speed of light. The Earth’s ionosphere sets a low-frequency limit to ground-based radio astronomy by reflecting extraterrestrial radio waves with frequencies below ν∼10 MHz (λ∼30 m), and the ionized interstellar medium of our own Galaxy absorbs extragalactic radio signals below ν∼2 MHz.

1 Introduction‣ Essential Radio Astronomy Chapter 1 Introduction 1.1 An Introduction to Radio Astronomy 1.1.1 What Is Radio Astronomy? Radio astronomy is the study of natural radio emission from celestial sources. The range of radio frequencies or wavelengths is loosely defined by atmospheric opacity and by quantum noise in coherent amplifiers. Together they place the boundary between radio and far-infrared astronomy at frequency ν ∼ 1 THz (1 THz ≡ 10 12 Hz) or wavelength λ = c / ν ∼ 0.3 mm, where c ≈ 3 × 10 10 ⁢ cm ⁢ s - 1 is the vacuum speed of light. The Earth’s ionosphere sets a low-freque

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