How are ALMA’s antennas connected?
Your eyes are connected to your brain through thick bundles of nerve cells. These optic nerves transfer the signals from your two retinas to your brain. Your brain processes the signals, and turns them into a nice three-dimensional view of your surroundings. Of course, it’s important that the signals from your left eye arrive at the same time as the signals from your right eye; otherwise your view of the world would become sort of blurry!
With ALMA, it’s more or less the same. ALMA’s eyes (the 66 antennas) are connected to its brain (a large supercomputer). Not through bundles of nerve cells, but through tens of kilometers of optical fibers. These fibers transfer the signals from ALMA’s receivers to the main computer, which is called a Correlator.
The Correlator is a huge computer. It fills a complete room in the Technical Building at the Chajnantor Plateau, at 5,000 meters altitude. This building is the second highest building in the world. The computer at which you are reading this has only a few processors, but the ALMA Correlator has 134 million processors. Together, they carry out 17 trillion calculations every second. So in a sense, ALMA’s brain is much more powerful than yours!
Why does ALMA need such a powerful brain? It’s because combining the signals from the 66 antennas is very complicated. It’s not like simply adding them together, as if you are adding 66 cups of water in a pan. If you would do that, the signals from ALMA’s 66 ‘eyes’ would get all mixed up. Yes, ALMA would still be very sensitive. After all, the 66 antennas have a total surface area as large as a football field, so together they collect a lot of millimeter waves. But the observatory would not be able to see very sharp.
Instead, you need to know precisely which signal came from which antenna. The locations of the antennas must be known accurately. And it’s also important to know very precisely when each signal was received. In fact, the timing error can not be larger than a millionth of a millionth of a second.
With all its accuracy and processing power, the ALMA Correlator does a great trick. It treats the 66 ‘eyes’ of the observatory as if they are small parts of one giant eye that can be many kilometers across. The result: ALMA is not only very sensitive, but it also has extremely sharp vision – even better than the Hubble Space Telescope.
If your brain could do the same trick – treating your two eyes as if they are parts of one giant eye six centimeters across – your eyesight would be twelve times better, and you would be able to read this text from a distance of more than twenty meters!