by Gene Allen
It took ten years and $1.3 billion to build the largest observatory on the planet, and it has an expected useful lifetime of only fifty years. The entire installation is rather bleeding edge technology.
ALMA is the Atacama Large Millimeter-submillimeter Array, a collection of 66 radio telescopes in northern Chile. The Chajnantor Plateau was chosen not just because it is among the highest and driest deserts in the world, but because it is the only such desert big enough for the array. It sits at 5,000 meters, which for us means 16,500 feet. Six of the scopes are in a rotating maintenance program at all times, and up to 50 of the 12 meter ones can be combined by interferometry after being moved about on the plain to a maximum separation of 16 kilometers, or about 10 miles. A subset of 4 12 meter and 12 7 meter dishes make up the Atacama Compact Array (ACA), used for wide field imaging. Two giant transporters were built to reposition the 100 ton scopes precisely among 192 concrete pads, all connected by power and fiber optic signal cables to the Array Operations Site (AOS) building, then down to the Operations Support Facility (OSF) at 9500 feet. The OSF is where the offices, telescope control, and most of the labs are located. There are residence halls, and a soccer field is being constructed. It is located nine miles off the highway, eleven miles south of San Pedro de Atacama, where we were spending a few days on a side trip, on our way to Antarctica..
Like most institutional observatories in Chile, ALMA offers very little opportunity for tourist visitation. They take only one bus load up to the OSF for about three hours, only on Saturday and Sunday. By the time we had determined an itinerary for our stay in San Pedro, both the primary and waiting lists online had been filled. Once I confirmed that the bus pickup location was a mere 12 minute walk from our hotel, it was obvious that I would try my luck.
I was in place a half hour early, and I was not the first. It was downright depressing how many people were gathering as the time approached. A woman appeared and gave a long speech in Spanish, then in English. By the time I understood that the clipboard being passed around was her “hope list,” establishing the boarding priority order for any of us extras, it had been handed to a young lady who had greeted me as she walked up. She held it up to me, acknowledging that I had been there first, and I just shrugged. How significant can it be? Maybe we all get to go, maybe none of us. I signed after her, as #7.
The reserved folks boarded, then the wait-listed. The coordinator climbed on to check, reporting 5 seats still available. As they were boarding, many of the rest began drifting away. I figured I had invested this much effort, I would wait until the bus actually drove off. Excitedly, she came back off and said she had made a mistake, there was one more seat. I said to her “The Amateur Astronomers Association of Princeton won’t hear about my visit after all.” She was apologizing, because she really did want to accommodate everyone. The girl who was #6 said, “You should take it, you were here first.” I replied, “It’s legitimately yours, and you are going to be so gracious as to give it up?” She said, “Well, it should have been yours. You go.” I regret that there was not time to get her name and thank her, at least here, for her kind and generous spirit.
ALMA captures electromagnetic waves just longer than infrared, which I rather inaccurately like to think of as even redder than infrared (real red is the other way, back toward visible). The ALMA Newsletter from April 2010 describes how it creates images in great detail, but for here I’ll just say it can see “cold and dusty molecular clouds, where star formation is occurring.” It’s not visible light, so they can image just as well in daytime, which is kinda neat.
Several things stood out for me during the tour. The transporters are so unique that they are named for the German designer’s children, Otto and Lore. It uses so much electricity to run the scopes’ cryostats and the data processing computers that it has its own multi-fuel turbine power station. It uses more water than the local aquifers can supply, so water has to be trucked 80 miles from Calama. In trying to better appreciate and convey the size of the installation, I tried to get a half dozen questions answered at once, but no one responded. I guess I’m stuck with submitting them one at a time to “ask an astronomer,” even though they don’t need an astronomer’s attention.
During the tour, I confirmed that there is one spot where the antenna array up on the Chajnantor Plateau can be observed from Route 27, through a saddle between two peaks. Afterwards, I learned that it can also be seen from Cero Toco, an even higher mountain nearby. Princeton University is a major collaborator in the Atacama Cosmology Telescope that sits up there at 16,900 feet. The fellow who was our private tour driver turned out to have been employed at that site, but was unable to work out any way to get me up there in the time we had. A trip up to see the array itself would have really put a bow on it. Anyone who is planning to go by there should contact me for details to make your visit even more complete.