4. From silver to silicon 00:00:24:18 00:00:27:23 400 years ago, when Galileo Galilei wanted to show others what he... 00:00:27:24 00:00:31:23 ...saw through his telescope, he had to make drawings. 00:00:31:24 00:00:35:04 The pockmarked face of the Moon. 00:00:35:05 00:00:39:08 The dance of the Jovian satellites. 00:00:39:09 00:00:41:02 Sunspots. 00:00:41:03 00:00:43:02 Or the stars in Orion. 00:00:43:03 00:00:45:15 He took his drawings and published them in a small book... 00:00:45:16 00:00:47:07 ...The Starry Messenger. 00:00:47:08 00:00:49:18 That was the only way he could share his discoveries... 00:00:49:19 00:00:51:07 ...with others. 00:00:51:08 00:00:55:14 For well over two centuries, astronomers also had to be artists. 00:00:55:15 00:00:57:23 Peering through their eyepieces, they made detailed... 00:00:57:24 00:00:59:22 ...drawings of what they saw. 00:00:59:23 00:01:02:00 The stark landscape of the Moon. 00:01:02:01 00:01:04:22 A storm in the atmosphere of Jupiter. 00:01:04:23 00:01:07:23 The subtle veil of gas in a distant nebula. 00:01:07:24 00:01:11:06 And sometimes they over-interpreted what they saw. 00:01:11:07 00:01:15:12 Dark linear features on the surface of Mars were thought to be canals... 00:01:15:13 00:01:18:20 ...suggesting civilised life on the surface of the red planet. 00:01:18:21 00:01:22:10 We now know that the canals were an optical illusion. 00:01:22:11 00:01:26:02 What astronomers really needed was an objective way to record... 00:01:26:03 00:01:30:09 ...the light collected by the telescopes without the information first having to... 00:01:30:10 00:01:33:10 ...pass through their brains and their drawing pens. 00:01:33:11 00:01:36:08 Photography came to the rescue. 00:01:37:15 00:01:40:01 The first daguerreotype of the Moon. 00:01:40:02 00:01:42:19 It was made in 1840 by Henry Draper. 00:01:42:20 00:01:46:04 Photography was less than 15 years old, but astronomers... 00:01:46:05 00:01:49:19 ...had already seized on its revolutionary possibilities. 00:01:49:20 00:01:51:24 So how did photography work? 00:01:52:00 00:01:56:02 Well the sensitive emulsion of a photographic plate contained... 00:01:56:03 00:01:58:07 ...small grains of silver halide. 00:01:58:08 00:02:01:01 Expose them to light, and they turn dark. 00:02:01:02 00:02:03:18 So the result was a negative image of the sky... 00:02:03:19 00:02:07:00 ...with dark stars on a light background. 00:02:07:01 00:02:10:12 But the real bonus was that a photographic plate can be... 00:02:10:13 00:02:12:22 ...exposed for hours on end. 00:02:12:23 00:02:15:15 When you take in the night sky with your own eyes... 00:02:15:16 00:02:18:13 ...once they're dark adapted, you don't see more and more... 00:02:18:14 00:02:21:06 ...stars just by looking longer. 00:02:21:07 00:02:24:04 But with a photographic plate you can do just that. 00:02:24:05 00:02:27:10 You can collect and add up the light over hours on end. 00:02:27:11 00:02:31:19 So a longer exposure reveals more and more stars. 00:02:31:20 00:02:33:01 And more. 00:02:33:01 00:02:34:04 And more. 00:02:34:05 00:02:36:05 And then some. 00:02:37:05 00:02:40:23 In the 1950s, the Schmidt telescope at the Palomar Observatory... 00:02:40:24 00:02:44:02 ...was used to photograph the entire northern sky. 00:02:44:03 00:02:48:24 Almost 2000 photographic plates, each exposed for nearly an hour. 00:02:49:00 00:02:51:21 A treasure trove of discovery. 00:02:51:22 00:02:56:00 Photography had turned observational astronomy into a true science. 00:02:56:01 00:03:00:10 Objective, measurable, and reproducible. 00:03:00:11 00:03:02:03 But silver was slow. 00:03:02:04 00:03:04:10 You had to be patient. 00:03:06:00 00:03:08:19 The digital revolution changed all that. 00:03:08:20 00:03:10:14 Silicon replaced silver. 00:03:10:15 00:03:13:10 Pixels replaced grains. 00:03:15:05 00:03:18:23 Even in consumer cameras, we no longer use photographic film. 00:03:18:24 00:03:22:11 Instead, images are recorded on a light-sensitive chip: 00:03:22:12 00:03:26:18 a charge coupled device, or CCD for short. 00:03:26:19 00:03:30:12 Professional CCDs are extremely efficient. 00:03:30:13 00:03:33:13 And to make them even more sensitive, they are cooled down... 00:03:33:14 00:03:36:22 . ..to well below freezing, using liquid nitrogen. 00:03:36:23 00:03:39:15 Almost every photon is registered. 00:03:39:16 00:03:44:14 As a result, exposure times can be much shorter. 00:03:44:15 00:03:48:10 What the Palomar Observatory Sky Survey achieved in an hour... 00:03:48:11 00:03:52:01 ...a CCD can now do in a few short minutes. 00:03:52:02 00:03:54:11 Using a smaller telescope. 00:03:54:12 00:03:57:00 The silicon revolution is far from over. 00:03:57:01 00:04:00:00 Astronomers have built huge CCD cameras with... 00:04:00:01 00:04:02:11 ...hundreds of millions of pixels. 00:04:02:12 00:04:05:05 And there's more to come. 00:04:07:00 00:04:11:11 The big advantage of digital images is that they're, well, digital. 00:04:11:12 00:04:14:17 They're all set and ready to be worked on with computers. 00:04:14:18 00:04:17:17 Astronomers use specialised software to process their... 00:04:17:18 00:04:19:19 ...observations of the sky. 00:04:19:20 00:04:24:00 Stretching, or contrast enhancing, reveals the faintest features... 00:04:24:01 00:04:26:14 ...of nebulae or galaxies. 00:04:26:15 00:04:30:03 Colour coding enhances and brings out the structures that... 00:04:30:04 00:04:32:13 ...would otherwise be difficult to see. 00:04:32:14 00:04:36:19 Moreover, by combining multiple images of the same object that... 00:04:36:20 00:04:39:08 ...were taken through different colour filters, one can... 00:04:39:09 00:04:43:06 ...produce spectacular composites that blur the boundary... 00:04:43:07 00:04:45:16 ...between science and art. 00:04:45:17 00:04:48:20 You too can benefit from digital astronomy. 00:04:48:21 00:04:52:21 It has never been so easy to dig up and enjoy the amazing... 00:04:52:22 00:04:54:18 ...images of the cosmos. 00:04:54:19 00:04:59:00 Pictures of the Universe are always just a mouse click away! 00:04:59:13 00:05:03:02 Robotic telescopes, equipped with sensitive electronic detectors... 00:05:03:03 00:05:06:18 ...are keeping watch over the sky, right now. 00:05:06:19 00:05:09:20 The Sloan telescope in New Mexico has photographed... 00:05:09:21 00:05:12:23 ...and catalogued over a hundred million celestial objects... 00:05:12:24 00:05:17:02 ...measured distances to a million galaxies, and discovered... 00:05:17:03 00:05:20:09 ...a hundred thousand new quasars. 00:05:20:10 00:05:22:23 But one survey is not enough. 00:05:22:24 00:05:26:08 The Universe is an ever-changing place. 00:05:26:09 00:05:30:03 Icy comets come and go, leaving scattered debris... 00:05:30:04 00:05:32:14 ...in their wake. 00:05:32:15 00:05:35:16 Asteroids zip by. 00:05:35:17 00:05:39:12 Distant planets orbit their mother stars, temporarily... 00:05:39:13 00:05:41:20 ...blocking part of the star's light. 00:05:41:21 00:05:47:17 Supernovas explode, while elsewhere new stars are born. 00:05:47:18 00:05:56:22 Pulsars flash, gamma-ray bursts detonate... black holes accrete. 00:05:56:23 00:06:00:16 To keep track of these grand plays of Nature, astronomers... 00:06:00:17 00:06:04:04 ...want to carry out all-sky surveys every year. 00:06:04:05 00:06:05:18 Or every month. 00:06:05:19 00:06:07:17 Or twice a week. 00:06:04:23 00:06:12:18 At least that's the ambitious goal of the Large Synoptic Survey Telescope. 00:06:12:19 00:06:18:07 If completed in 2015, its three- gigapixel camera will open up... 00:06:18:08 00:06:21:00 ...a webcam window on the Universe. 00:06:21:01 00:06:24:22 More than fulfilling astronomers' dreams, this reflecting telescope... 00:06:24:23 00:06:30:00 ...will photograph almost the entire sky every three nights.