Runcell

The first image of a black hole was taken by a telescope the size of the Earth. But no mirror or dish on Earth is that big. So how was it done? This video starts with the simplest telescope and follows one idea all the way up. A magnifying glass makes small print bigger, but it turns the distant Moon into a blur. Add a concave lens in front of your eye and the Moon jumps closer: that is the telescope Galileo built in 1609. Then come the refracting telescope and Newton's reflecting telescope. What does a bigger mirror actually get you? Not magnification. That comes from the mirror's curve and the eyepiece. A bigger mirror collects more light, so you can see fainter, more distant things. A wider mirror also sees sharper, splitting stars that sit close together. Make it twice as wide and you collect four times the light and see details twice as fine. So for 400 years, astronomers have kept making mirrors bigger, and kept hitting limits: 4 cm: Galileo sees craters on the Moon and four moons circling Jupiter. 1 m: a lens can only be held by its edge, and any bigger it sags. 5 m: a mirror can be supported from behind. 8 m: a single piece of glass reaches its limit. 10 m and 39 m: mirrors built from 36 and 798 segments (the 39 m one is still under construction). 10,000+ km: eight radio telescopes around the world work as one Earth-sized telescope. Why can a mirror be built from pieces, or even spread apart? Cover part of a mirror and the image doesn't lose a corner. It only gets dimmer. With visible light, the pieces have to bring their light together in real time, so they stay within a few hundred meters of each other. Radio dishes can record the waves against atomic clocks and combine the recordings later, which is how the Event Horizon Telescope spans the planet. Chapters 0:00 A telescope the size of the Earth 0:10 One lens, then two: Galileo's telescope 0:34 Refractors and reflectors 0:50 What a bigger mirror really gets you 1:30 Making mirrors bigger: 1 m, 5 m, 8 m 1:58 The limit, and covering part of a mirror 2:19 Mirrors made of pieces 2:40 Why radio dishes can span the Earth 2:51 The Earth-sized telescope 3:05 The black hole image, and what comes next Notes The light-path diagrams, the covering experiment and the quasar starfield are illustrations. The Galilean-type telescope shown is a museum piece in Paris, not Galileo's own instrument. A fleet of mirrors in space that could show the continents of another Earth is a proposed concept, not a planned mission. Sources: Museo Galileo (catalogue records and Sidereus Nuncius); the Royal Society and the Science Museum Group (Newton's reflecting telescope); Yerkes Observatory; Caltech Palomar Observatory (the 200-inch Hale Telescope); W. M. Keck Observatory; European Southern Observatory (ELT, VLT and VLT Interferometer, eso9845d, eso1907); the CHARA Array; Event Horizon Telescope press release, 10 April 2019; Kardashev, Kovalev & Kellermann, "RadioAstron: An Earth-Space Radio Interferometer with a 350,000 km Baseline" (2012); Labeyrie, hypertelescope concept (ESA, 2003); OpenStax Astronomy and College Physics (light gathering and the Rayleigh criterion); NASA. Images: First image of a black hole: Event Horizon Telescope Collaboration (ESO eso1907a, CC BY 4.0) Galilean telescope: Rama, Musée des Arts et Métiers, Paris (Wikimedia Commons, CC BY-SA 3.0 FR) Galileo's drawings of the Moon: Galileo Galilei, 1610 (public domain) The Moon from Yerkes Observatory: Keystone View Company, before 1917, Museum of Photographic Arts (no known copyright restrictions) The Palomar 200-inch mirror: Los Angeles Daily News, UCLA Library (CC BY 4.0) NGC 1232: European Southern Observatory (eso9845d, CC BY 4.0) The four planets of HR 8799: Jason Wang (Caltech) / Christian Marois (NRC Herzberg), W. M. Keck Observatory (CC BY 4.0) Webb's First Deep Field: NASA, ESA, CSA, STScI (public domain) Earth: NASA Earth Observatory, Blue Marble (public domain) All other visuals are original.