Telescope characteristics and radio astronomy

Radio astronomy began in 1932 when Karl Jansky detected radio noise from the direction of the Milky Way’s centre; in 1937 Grote Reber built the first parabolic dish. A radio telescope is a parabolic antenna (dish) with a sensitive receiver: it focuses radio waves and turns them into an electrical signal. Radio waves are ≈ 10³–10⁷ times longer than visible light, so by θ ≈ 1.22 λ / D a single dish has poor resolution; to overcome this many antennas are combined into an interferometer, whose baseline can be thousands of kilometres (VLBI). Large radio telescopes: Effelsberg (100 m), Green Bank (100 m), FAST (China, 500 m, 2016), and the multi-dish VLA (27) and ALMA (66, Chile). The 21 cm (1420 MHz) line of atomic hydrogen lets us study the structure of galaxies. In 2019 the Event Horizon Telescope collaboration published the first image of the black hole at the centre of galaxy M87, and in 2022 of Sagittarius A* at the centre of the Milky Way.