Imagine a trio of aerobatic aircraft. Over the years they’ve gotten very good at their routine. But they want to add another five or six or seven members. They also want to upgrade from propeller planes to jets, with custom engines and digital avionics. And they plan to do all of this upgrading and expanding while simultaneously flying their planes in a once-a-year show, a qualifying run for the equivalent of the Olympics.
This might not be the best analogy for the Event Horizon Telescope‘s 2015 observing campaign, which took place on five nights in late March. No one was going to fall out of the sky in a ball of fire if something went wrong. They didn’t have to nail the routine every night. If they managed to shake down the upgraded, expanded array and bank one good observing night, with clear weather and fully functioning telescopes at every site, the run would be a success. But they were fielding new equipment and new telescopes simultaneously during a once-a-year window of opportunity. And the event they’re building up to–taking a picture of a black hole–is pretty Olympian.
We won’t know for a while whether they banked that one all-important good observing night. But they certainly did a lot of shaking down.
Some background: For years, the Event Horizon Telescope has been operating as a trio of sites in Hawaii, Arizona, and California. (It’s a little more complicated than that–other sites have joined in here and there, and there are actually two telescopes on Mauna Kea that participate–but basically, until this year the EHT was a triangle.) Its goal is to image Sagittarius A*, the 4 million-solar-mass black hole at the center of the Milky Way, an achievement that could have profound implications for our understanding of the universe. To do that, the EHT needs to grow into a truly worldwide array. That’s a matter of adding the following telescopes to the array:
- Large Millimeter Telescope (LMT) near Puebla, Mexico
- Atacama Large Millimeter Array (ALMA) in northern Chile
- Atacama Pathfinder Experiment (APEX) telescope in northern Chile
- South Pole Telescope (SPT)
- IRAM 30 meter dish in Spain
- Plateau de Bure interferometer in France
(Eventually the plan is to add the Greenland Telescope, but it hasn’t yet been built, so we’ll set it aside for now.)
Several months ago, it seemed possible that most of these telescopes might participate in the 2015 observing campaign, but that was always iffy. ALMA can’t join until the ALMA Phasing Project–a big technical upgrade led by EHT scientists at MIT’s Haystack Observatory–is finished, and that work is ongoing. Until very recently, the Large Millimeter Telescope in Mexico still didn’t have a receiver that operates at the millimeter wavelengths the EHT needs. Until January, the South Pole Telescope didn’t have the right kind of receiver, either. And every site in the array, including the American stations that the EHT has been using for years, had to be outfitted with new, high-bandwidth signal processors and recorders capable of inhaling the huge amount of digitized light it takes to see down to the event horizon of a black hole.
That’s a lot of technical work to do in a short amount of time. And the reason March 2015 was a hard deadline is that the EHT can really only observe once a year, when the weather is (in theory) good enough at every site and the Earth is oriented properly with respect to the galactic center.
So here’s what happened.
Over the past year, every site in the array got some sort of overhaul involving many crates filled with hand-built signal processors, data recorders, and other electronics.
One of the two European stations–the IRAM 30-meter dish–participated in the observations, but the Plateau de Bure interferometer is awaiting some technical upgrades, so it sat this one out.
The South Pole Telescope got the receiver it needed–more about that here–but it couldn’t participate in this observation, because once the receiver was installed and confirmed to be running, the team had to ship it back to Tucson for upgrades that will allow it to work at even higher frequencies. (Shipping a receiver to the South Pole, installing it, then packing it back up and sending it to the U.S. for upgrades might sound insane, but by the logistical standards of Antarctic science it is apparently pretty standard. It’s not easy to do delicate work on the guts of a superconducting machine at the South Pole. Also, because of other upgrades happening at the South Pole Telescope, they would have had to reinstall the receiver next year anyway.)
APEX was all set to participate, but its new 1.3 mm receiver malfunctioned, and the company in Sweden that built the instrument couldn’t fix it in time for the observation.
For various reasons, including the fact that the Alma Phasing Project is still ongoing, ALMA didn’t join.
The Large Millimeter Telescope’s 1.3 mm receiver came together just under the wire. EHT scientists, led by Gopal Narayanan of the University of Massachusetts-Amherst, built a receiver and got it to Mexico barely in time for the spring observation. The reason they had to cobble it together is that they didn’t have funding to build the receiver until late last year. The reason it took so long to get it to Mexico involves, in part, the ridiculous amount of snow that Boston got this year; weather delayed the final testing and shipping of the instrument by weeks.
For all these reasons, this observing run was mostly a shakedown session with two goals: 1) Get all the new receivers and recorders at every station working properly, and 2) Add the Large Millimeter Telescope (LMT) to the array.
Adding a single telescope to the array might not sound like much, but bringing in the LMT is a big deal. First off, the LMT is huge. It’s a 50-meter dish, with 32 meters of surface currently polished and tuned with enough accuracy to work at 1.3 millimeters. (The rest of the dish’s surface should be finished by this time next year.) Second, the LMT, a joint Mexican-American project supported by both CONACYT and U.S. funders, including the NSF, happens to be located on a very convenient patch of the globe. Think of the individual telescopes in the EHT array as single silvered dots on a planet-size virtual mirror. The more dots you have, the better. But you also need those dots to be in the right places; you don’t want big holes in your collecting area. And the LMT fills an important hole. (In astronomer jargon, the LMT fills important holes in the UV plane.)
I flew down to Mexico for the first few days of the observation and spent several nights at the LMT. (I’m tempted to describe the look and the feel of this strange, remote place, but I’ve already told a long LMT story on this blog, and there’s a lot here to cover.) Inevitably, there were problems. The mirrors that reflect light into the receiver were slightly misaligned, so that when the astronomers tried to calibrate the telescope by looking at Jupiter, the planet was much dimmer than it should have been–not all of the light was getting to the receiver. The astronomers fixed that problem through a complicated procedure involving lasers and liquid nitrogen. But there was another issue: a bizarre wiggle in the data produced by the receiver. That little glitch, which came from deep inside the receiver, made it difficult to “point” the telescope–that is, to fix the dish precisely on the tiny, dim objects under study. This was a much bigger problem than the mirror misalignment. It took a few days of frantic work for the astronomers to find a way to bypass the glitch and point the telescope accurately.
Finally, one night toward the end of the 10-day run, the skies were clear and the bugs eradicated at the sites in Hawaii, California, Arizona, and Mexico. The data recorded that night is now back at Haystack Observatory, where a supercomputer will synthesize it all and look for common detections among the sites. With luck, the Event Horizon Telescope will have gotten its closest look yet at Sagittarius A*, the black hole at the center of the Milky Way.
I’ll post an update as soon as the correlator has produced results. More soon….