The Atacama Large Millimetre/Sub-Millimetre Array (ALMA)

Our flight to northern Chile and stay in San Pedro de Atacama was designed so we could acclimatise to the high altitude and arid conditions of the high desert before visiting the radio observatory at 5050 m on the Atacama plateau.  ALMA is the world’s most sophisticated observatory at these wavelengths, a truly collaborative project between Europe, Japan, America and Chile.  It works by having fifty 12-m antennae (the Main Array) with variable separations in between.  The different baselines allow you to take a Fourier Transform of the sky, correlating the signals from each antenna to provide a spatial resolution far superior to what you can achieve from a single dish in isolation.  The sensitivity to a particular spatial scale depends on the length of the baseline.  Short baseline configurations (150-m) provide access to large spatial scales; long baselines (up to 15 km) provide access to the smallest spatial scales. Twelve additional 7-m antennae (the Compact Array) provide very short baselines for the largest spatial scales.

Within each antenna is a series of receivers, or bands, which determine what wavelengths can be observed.  At the moment bands 3-9 are available to users, providing wavelengths from 420 µm to 3.6 mm (84-720 GHz).  This should extend up the 950 GHz when band 10 is offered, and maybe down to 30 GHz with future receiver development.  Measurements in these spectral bands, at a variety of spectral resolutions, are then fed by cables to the main correlator building a few metres away from the antennae, using the Fourier Transform to assemble an image of the sky with unprecedented spatial resolution.

The ALMA site offers the best observing conditions in Chilean winter (July to November), so the more challenging configurations are used then.  In February the conditions are usually hazardous with extreme snowfall, meaning that the antennae must be oriented so that snow does not accumulate inside the dishes.  Conditions below the snow line can be so wet that flash flooding can destroy the access roads.  Thankfully, by the time of our visit in March, conditions were cold and clear again and excellent for some astronomy tourism.

Extreme Tourism at 5 km 

On Saturday morning we met a bus at 7am for the drive out to the OSF (Observer Support Facility) at an altitude of 2500m.  The sun was rising over the volcano to the east as we drove the dirt road to the facility, a cluster of buildings featuring the main control room and data banks, in addition to the hangers and workspaces of the contractors from Japan, the US and Europe responsible for constructing and delivering the 66 antennae (now empty as their work is done).  Construction was still taking place for a permanent visitor quarters, with temporary buildings housing the astronomers.  We had to each undergo a compulsory medical exam (blood pressure and O2 levels) and safety video, being supplied with small oxygen canisters to use should we feel dizzy at high altitude.  This is a serious medical screening - one of our number didn’t pass and had to remain at the OSF.

We ascended the switch-backed road through the mountains, watching as the dry slopes gave way to some green vegetation and cacti, allowing the grazing donkeys and llama (vicuna) to survive despite the seemingly hostile conditions.  The landscape was not as volcanic or young as that on Mauna Kea, this is a more ancient geology.  We all started to feel lightheaded, but cheered as we passed the 4200-m mark (the height of the Mauna Kea observatories).  For many of us, this marked the highest point we’ve ever been to in our lives.  As we crossed the mountain pass, the plateau opened up before us and the ALMA array came into view.  A small Japanese observatory could be seen on one of the high peaks, which must be one of the highest manned observatories in the world.

We had about half an hour to wander amongst the antennae, which was in a compact configuration after the February snows and undergoing engineering work to prepare for more science.  Eric Villard served as an excellent tour guide, showing the differences between the US and European antennae designs.  As we watched, one of the antennae rotated around silently, controlled by some unseen operator down at the OSF.  The thin air (0.5 bar) and lack of O2 at 5050 metres above sea level does strange things to the brain, an almost drunken experience as we posed for photos with the array in the background.  All of us took occasional puffs from the oxygen canisters if we felt any dizziness, but thankfully I didn’t experience any of the headaches or nausea sometimes associated with high altitude.

We then went inside the correlator building, seeing the banks and banks of computers and hard drives required to bring together the signals from each of the individual antennae.  Then it was time to begin our 45 minute descent back to the OSF for lunch and then on to Calama, drinking in the thicker atmosphere and feeling tired.  It had been a tremendous experience, not only because I now know more about the challenge of radio interferometry for astronomy, but also because of the extreme environment we’d been lucky enough to visit.  Very few humans get this opportunity, so I’m extremely grateful to ESO and the organisers of #planets2015 for the chance!