ALMA: Extreme, Precision Astronomy in the Desert
The Atacama Large Millimeter/Submillimeter Array, or ALMA, is nearing completion after over 30 years of planning and collaborating among astronomers and engineers from several nations. The completed array will consist of 66 antennas and two supercomputers, called correlators, at the backend to collect the signals and make the array function as one large telescope.
On March 12th and 13th, 2013, over a hundred journalists descended upon the array along with politicians, scientists, engineers, and other VIPs to celebrate the official inauguration of the array and tour its facilities. I was there along with 11 other journalists from around North America as guests of the National Radio Astronomy Observatory.
Overview of the telescope's technical capabilities, science goals, and history were given by (from left to right) Michael Thorburn, Head of the ALMA Department of Engineering, Pierre Cox, incoming ALMA director, Thijs de Graauw, current ALMA director, Al Wootten, ALMA Program Scientist for North America, and Ewine van Dishoeck, Professor at Leiden University and former ALMA board member.
The ALMA control room is the hub of activity at the "low site" or Operations Support Facility. From here, telescope operators manage and control all array operations and conduct observing runs. Astronomers who win time on the telescope through a merit-based proposal process do not actually travel to the site or control the telescope in real time. Instead, as with other radio interferometers, they prepare observing scripts based on what science they want to achieve, and their scripts are then scheduled and run by ALMA staff. The data is then made available to the astronomer for a proprietary period of one year, when it is then released to the public.
Everyone gets into the spirit of ALMA, which in Spanish actually means "soul," as an art contest was held for local children. Featured here are some of the submissions and several winners in the six to nine year old group.
54 of the final 66 antennas were at the "high site" or Array Operations Site, at an altitude of 16,500 feet. The 12-meter dishes are contributed by partners in Europe, North America, and East Asia, whereas several smaller, 7-meter dishes from East Asia make up a compact array in the center. The completed array will eventually have the capability of expansion by moving the antennas to different concrete "pads" spread around the Chajnantor Plateau. With a baseline, or distance between antennas, of up to 14 kilometers, ALMA will be able to obtain resolution ten times better than that of the Hubble Space Telescope.
Reporters were allowed a peek at the massive correlator behind glass in the second highest building in the world. The correlator is the computer that brings together signals from all the antennas so that the astronomer can make an image using the full array. We received a thorough explanation of its working from correlator engineer Alejandro Saez who has spent time constructing in in Charlottesville, Virginia, and at the high site in Chile. The correlator has the processing power of 3 million laptops as it has to make calculations for up to 1,125 antenna pairs billions of times per second.
The antennas actually put on a show for us during our three hour tour of the high site, slewing, or moving, back and forth. These antennas are built for perfection, or as close to it as one can come. These sturdy, yet flexible machines must maintain a dish surface accuracy of the width of a human hair and pivot back and forth between a target source and calibrator source on the sky every ten seconds.
Taking a hundred or so journalists to a site at 16,500 feet (5000 meters) altitude is quite risky business. Everyone must pass a basic fitness exam to be cleared for access, though employees must endure a much more rigorous process involving a stress test and heart monitoring to work on site. A team of diligent paramedics were there to hand out oxygen bottles and check for signs of altitude sickness. Here, I got my pulse checked in an ambulance after running around like an excited child a bit too much.
The "front end" of a radio telescope is the place where radio light that is collected by the dish is received, amplified, and transferred to the next stage for digital processing. The large blue drum is a cryostat and holds the most sensitive electronics in the telescope at a cold 4 Kelvin using liquid helium. The silver circles are the actual windows into the feedhorms, the first stage where radio light passes into the front end. Don't think those metal plates are transparent? They are to the radio light that ALMA receives.
Just one of ten sets of electronics that can fit inside those blue drums are on display here. Various stages of the system are cooled to difference temperatures, and engineers needs to wear special gloves and shirts when handling these so that they do not impart a spark of static electricity that could ruin a sensitive (and expensive) piece of equipment. The superconducting receivers used by ALMA are state-of-the-art and were developed specifically for this purpose.
How do you move a giant antenna? You build a giant antenna mover. The on-site guests were treated to an amazing site when one of the two antenna crawlers rumbled down a huge dirt road to pick up one of the dishes that is still under testing. Otto and Lore, as the crawlers are named, have to pick up each 100-ton antenna and place them down on their pads with millimeter accuracy. They move the antennas from the Operations Support Facility up to the high site and back and will be used to change array configurations to change the telescopes resolving power.
The Atacama Large Millimeter/Submillimeter Array, or ALMA, is already producing amazing science results. To see the telescope up close at an altitude of 16,500 feet (5,000 meters) is even more incredible. This high-precision instrument on top of the world is truly one of the most impressive sights that I have ever seen.
So what’s the big deal with a “millimeter and submillimeter” telescope anyway?
ALMA is the most sensitive instrument (by far) to probe this region of the electromagnetic spectrum just a bit longer in wavelength than infrared, yet still quite high energy for most radio astronomers.
It has a special power to see an unbiased sample of the universe. That is, by a combination of an increase of star formation (and thus infrared emission) in the galaxies in the early universe and the way that light is redshifted by the expansion of the universe, you can see a whole swath of the history of galaxies in this band. However, it has traditionally been a difficult place to work since the water molecules in our atmosphere absorb and scatter much of the submillimeter light coming from space.
So, to some of the highest, driest mountain peaks we go.
I came to the Atacama Desert as a guest of the National Radio Astronomy Observatory along with several other science writers from around the United States. At the Operations Support Facility, located at an altitude of 9,500 feet (2,900 meters), we joined an even larger host of journalists from around the world to get a special tour of this remote facility.
The altitude was already wearing on a few of us that are used to sea level, especially when lugging around laptops and camera bags. We had a safety briefing before our trip to the high site where we were instructed on the use of our oxygen bottles and informed that a team of paramedics would be traveling with us to the high site, or Array Operations Site. This was about to get real.
Despite the occasional dizziness and mild headache once we reached the array, I actually jumped up and down and squealed with excitement upon seeing it in person. There were 54 dishes on site from North America, Europe, and East Asia, all built to the same precise performance specifications but each looking a little bit different. The surface accuracy of the gleaming 12-meter wide dishes is the width of a human hair, and the drives and motors that move them must point to an object with 0.6 arcseconds of accuracy. (That’s like pointing accurately at a single person in Charlottesville, Virginia, from St. Louis. Trust me, that’s a LONG drive.) Seeing the arrays in person was… beautiful.
That’s the other thing about observing in what we call “high frequencies” for radio astronomy. You are allowed a margin or error, but that is only a fraction of the wavelength of light that you are observing. Traditionally, surface and positions accuracies had to be good down to a centimeter, even a few millimeters. Very long wavelength projects can get away with even more, as I learned from my experience building telescopes that collected light with 2 meter wavelengths. (The very first array was more or less paced out. Seriously.) But with the short wavelengths viewed by ALMA, micron accuracy of the instruments must be achieved, and this is quite a technological hurdle.
The array even put on a bit of a show for us, slewing, or moving back and forth to point to different areas of the sky. At first, some of the moves were part of usual on-site testing, but then they really whirled them around for us to get video and pictures. I do not say “whirled” lightly as these dishes moved FAST. The 100-ton dishes can turn completely around in just a minute and do so extremely quietly.
Groups of eager journalists filed into a building to see the workhorse supercomputer called the correlator. It had been built in sections in Charlottesville, Virginia, and all connected at the array site by engineers, such as Alejandro Saez, who happily answered our questions. With a processing power of 3 million laptops, the correlator brings all of the antenna signals together from the 12-meter dishes so that a data product can be given to the astronomers. A second correlator is being built by the National Astronomical Observatory of Japan to serve the 7-meter dishes in the center of the array that are there to image the most diffuse radio emission on the sky.
The excitement and jumping around continued until I was well out of breath, so I’d take a hit of oxygen every once in a while. I never did get my blood oxygen tested, but the paramedics noted that my pulse was a bit high. Others on site had a bit more trouble and were given tanks and nose plugs to help with the altitude sickness. These are also worn by staff who work at the high site, and they are restricted to only spending 6 hours a day there.
One of the blue cryostats that house the “front end” electronics.
Back at the OSF, a few of us managed to get a tour of one of the labs where the electronics, or “guts” of the telescope are tested. I recognized the familiar blue drums of the cryostat that had been assembled in Charlottesville when I was a grad student, but the engineers had taken it apart so we could see inside.
The radio signals pass into the cryostat, cooled down to 4 Kelvin by liquid helium, and onto electronics that amplify the signal and change its frequency so that it can be handled by digital electronics. These amplifiers and receivers are what actually measure the amount (and phase) of radio light being collected by the telescope and are some of the most sophisticated in the world as they were designed specifically for this application.
My personal favorite device, however, was the robot arm that moves around in front of the receivers when the astronomer needs to calibrate the telescopes in order to make accurate measurements.
There is no doubt that I am a radio astronomy nerd. I’ve worked at several different observatories and delight in the mechanics of operation. However, this was a spectacle that wowed everyone in attendance. The technology, power, and raw human ingenuity and passion that go into such a project enough to move anyone to fall in love, as I finally did, with ALMA.
Billions and billions of thanks to the National Radio Astronomy Observatory, specifically to John Stoke, Tania Burchell, and Charles Blue, for giving me the opportunity to take this amazing trip!
All photos by the author, CC BY-NC-SA. If you are interested, please also check out a short video I made about the trip!