My bedroom in the second floor of the castle has an armillary sphere, a model of the Solar System that includes Uranus, the planet William Herschel discovered in 1871. I was delighted to find the antique, it is not only an exquisite work on wood, paper and metal, but also inspiring. I cannot think of a better object to be on top of my dresser, to be seen several times a day, because I came to Civitella to write a book about astronomy and telescopes . Was it left there on purpose? Or it simply has inhabited that room for years? Either way it is perfect for my work.
Excerpt from Our Next Universe
“Sorry, but you need to see this,” says Fabien Patru, grabbing a blank sheet of paper from his desk, scribbling something on it, and suddenly jumping out of his chair. Startled, I too stand up. Fabien crosses his small office, walks four or five meters into the hall, and turns to face me, holding the sheet of paper just below his chin. “What do you see?” he asks me. At first I see nothing. Fabien waits. Then something. “A tiny black dot,” I say, “on the upper left corner.” Satisfied, he walks slowly towards me, still holding up the sheet of paper. He stops at the doorframe. “And now?” he asks. “Oh”, I say, “It’s not one dot, there are two of them!”
Fabien is a young astronomer turned engineer. So is Jacopo Antichi, whose desk is also in the small office they both share. There is barely room for me, and not a single window in this office. Light comes in through the hall, through the bulb hanging from the ceiling, and from three computer screens on Jacopo’s desk and one on Fabien’s. We are at the Arcetri Astrophysical Observatory, a set of buildings on top of a hill in the outskirts of Florence, surrounded by pine woods. The hill is steep. It takes about half an hour to walk all the way up to Arcetri from the magnificent Villa del Poggio Imperiale, a palace the Medici took hold of, which also for some time belonged to Napoleon’s sister and which today houses a public school. But I didn’t walk. Afraid of getting lost in the winding road, or soaked by the light rain, I took a taxi right after arriving by bus at the foot of the hill, where the Porta Romana stands.
The demonstration with the dots, which Jacopo watched from his desk with an expression of amusement, was intended to explain a key concept in optics: resolution. It refers to how close two objects can be next to each other and still be seen separately with our eyes or an optical contraption, such as a pair of binoculars or a telescope. Take stars, for example. Some stars appear close together in our skies (like Alcor and Mizar, in the Big Dipper), and some are physically close, in two-star systems called binaries (there are even three- and four-star systems). We cannot bring the star, or a system of two stars, next to us. If we want to see them with a better resolution, we would need to increase the size of our pupils to allow more light rays to enter and form a sharper image on the retina. We cannot do that either. But it is possible to have larger telescopes, increasing the size of their pupils. The pupils of professional telescopes are usually mirrors –the primary mirror captures the light and sends it to another, smaller mirror, which in turn directs the light to the instruments that will record and analyze it. If you gather more light with a mirror, the end result is like zooming in on the object. “This is the reason why we want to make the telescope bigger and bigger”, says Fabien.
But that’s not all there is to it. The mortal enemy of good resolution is the Earth’s atmospheric turbulence, produced by the wind, and by temperature differences. Turbulence makes stars twinkle, or rather, makes us see them twinkle. Fabien offers an analogy. “It’s like water”, he says. If you look into a pool of still water, you can see all the way to the bottom, “but as soon as someone jumps in, you can’t see anything.” What about the light from the stars? Back at his desk, he draws light waves coming from a star. A star emits light in all directions. Some of them will move in the direction of the Earth. The star is so far away from us, that light waves coming from different parts of its surface look like they were emitted at the same time. The tips, or fronts, of these light waves are aligned, like an advancing squad, forming what astronomers call a flat wave front. But once they encounter our planet’s atmosphere, their behavior changes, this wave front becomes messy. The neat rows of marching soldiers break formation, each soldier moving slightly slower or faster than the others. The wave front is no longer flat. The light waves don’t hit the mirror of the telescope at the same time, which is enough to give a fuzzy, blurred image of the star. “Horrible,” I say. “Yes, it is horrible,” agrees Fabien in his heavy French accent, “because if you have two stars you cannot tell them apart.” Other celestial objects might lose their identity in the same way. The transforming power of turbulence can make a resplendent galaxy with billions of stars populating its spiral arms look like a blurry blob.
Jacopo turns his eyes away from the computer screens and intervenes, moving his chair towards Fabien’s desk. He has a soft voice, with a slight Italian accent. I wonder if I am about to see another demonstration. But Jacopo doesn’t stand up. Instead he explains that the larger the optic area to capture light in a telescope, the bigger the effect of the turbulence, because there is more air inside the telescope.
What is the point, then, in building bigger telescopes, something that Fabien clearly yearns for?
The point is that it is possible to cancel a high percentage of the effects turbulence has on astronomical images by using what is called Adaptive Optics Systems. These systems can tame the light waves from celestial objects once they have entered the turbulent air in a telescope, reshaping the distorted wave front, making it flat again. Synchronizing the soldiers. It’s like the telescope was in outer space, orbiting our planet, far from its whimsical atmosphere. That is already happening in the biggest telescopes on Earth, like the Gemini I on Mauna Kea, Hawaii, or the Clay Magellan, in Chile.
The next generation of telescopes, three giants, each the size of a cathedral, also referred to as “gargantuan” and “behemoths” and “extremely large,” four times bigger than today’s largest, only makes sense if they are equipped with adaptive optics systems. The job of Fabien and Jacopo is to help create those very systems. One would think they deserve a larger office, or at least a window.