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4. From silver to silicon
“400 years ago, when Galileo Galilei wanted to show others what he saw through his telescope, he had to make drawings. The pockmarked face of the Moon. The dance of the Jovian satellites. Sunspots. Or the stars in Orion. He took his drawings and published them in a small book ‘The Starry Messenger’. That was the only way he could share his discoveries with others. For well over two centuries, astronomers also had to be artists. Peering through their eyepieces, they made detailed drawings of what they saw. The stark landscape of the Moon. A storm in the atmosphere of Jupiter. The subtle veil of gas in a distant nebula. And sometimes they over-interpreted what they saw. Dark linear features on the surface of Mars were thought to be canals suggesting civilised life on the surface of the red planet. We now know that the canals were an optical illusion. What astronomers really needed was an objective way to record the light collected by the telescopes without the information first having to pass through their brains and their drawing pens. Photography came to the rescue.
“The first daguerreotype of the Moon. It was made in 1840 by Henry Draper. Photography was less than 15 years old, but astronomers had already seized on its revolutionary possibilities. So how did photography work? Well the sensitive emulsion of a photographic plate contained small grains of silver halide. Expose them to light, and they turn dark. So the result was a negative image of the sky with dark stars on a light background. But the real bonus was that a photographic plate can be exposed for hours on end. When you take in the night sky with your own eyes once they're dark adapted, you don't see more and more stars just by looking longer. But with a photographic plate you can do just that. You can collect and add up the light over hours on end. So a longer exposure reveals more and more stars. And more. And more. And then some.
“In the 1950s, the Schmidt telescope at the Palomar Observatory was used to photograph the entire northern sky. Almost 2000 photographic plates, each exposed for nearly an hour. A treasure trove of discovery. Photography had turned observational astronomy into a true science. Objective, measurable, and reproducible. But silver was slow. You had to be patient.”
The digital revolution changed all that. Silicon replaced silver. Pixels replaced grains. Even in consumer cameras, we no longer use photographic film. Instead, images are recorded on a light-sensitive chip: a charge coupled device, or CCD for short. Professional CCDs are extremely efficient. And to make them even more sensitive, they are cooled down to well below freezing, using liquid nitrogen. Almost every photon is registered. As a result, exposure times can be much shorter. What the Palomar Observatory Sky Survey achieved in an hour a CCD can now do in a few short minutes. Using a smaller telescope. The silicon revolution is far from over. Astronomers have built huge CCD cameras with hundreds of millions of pixels. And there's more to come.
“The big advantage of digital images is that they're, well, digital. They're all set and ready to be worked on with computers. Astronomers use specialised software to process their observations of the sky. Stretching, or contrast enhancing, reveals the faintest features of nebulae or galaxies. Colour coding enhances and brings out the structures that would otherwise be difficult to see.
“400 years ago, when Galileo Galilei wanted to show others what he saw through his telescope, he had to make drawings. The pockmarked face of the Moon. The dance of the Jovian satellites. Sunspots. Or the stars in Orion. He took his drawings and published them in a small book ‘The Starry Messenger’. That was the only way he could share his discoveries with others. For well over two centuries, astronomers also had to be artists. Peering through their eyepieces, they made detailed drawings of what they saw. The stark landscape of the Moon. A storm in the atmosphere of Jupiter. The subtle veil of gas in a distant nebula. And sometimes they over-interpreted what they saw. Dark linear features on the surface of Mars were thought to be canals suggesting civilised life on the surface of the red planet. We now know that the canals were an optical illusion. What astronomers really needed was an objective way to record the light collected by the telescopes without the information first having to pass through their brains and their drawing pens. Photography came to the rescue.
“The first daguerreotype of the Moon. It was made in 1840 by Henry Draper. Photography was less than 15 years old, but astronomers had already seized on its revolutionary possibilities. So how did photography work? Well the sensitive emulsion of a photographic plate contained small grains of silver halide. Expose them to light, and they turn dark. So the result was a negative image of the sky with dark stars on a light background. But the real bonus was that a photographic plate can be exposed for hours on end. When you take in the night sky with your own eyes once they're dark adapted, you don't see more and more stars just by looking longer. But with a photographic plate you can do just that. You can collect and add up the light over hours on end. So a longer exposure reveals more and more stars. And more. And more. And then some.
“In the 1950s, the Schmidt telescope at the Palomar Observatory was used to photograph the entire northern sky. Almost 2000 photographic plates, each exposed for nearly an hour. A treasure trove of discovery. Photography had turned observational astronomy into a true science. Objective, measurable, and reproducible. But silver was slow. You had to be patient.”
The digital revolution changed all that. Silicon replaced silver. Pixels replaced grains. Even in consumer cameras, we no longer use photographic film. Instead, images are recorded on a light-sensitive chip: a charge coupled device, or CCD for short. Professional CCDs are extremely efficient. And to make them even more sensitive, they are cooled down to well below freezing, using liquid nitrogen. Almost every photon is registered. As a result, exposure times can be much shorter. What the Palomar Observatory Sky Survey achieved in an hour a CCD can now do in a few short minutes. Using a smaller telescope. The silicon revolution is far from over. Astronomers have built huge CCD cameras with hundreds of millions of pixels. And there's more to come.
“The big advantage of digital images is that they're, well, digital. They're all set and ready to be worked on with computers. Astronomers use specialised software to process their observations of the sky. Stretching, or contrast enhancing, reveals the faintest features of nebulae or galaxies. Colour coding enhances and brings out the structures that would otherwise be difficult to see.
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