How to Take a Picture of a Black Hole

First, build a telescope the size of planet Earth

ALMA Prototype Radio Antennas

….. The Event Horizon Telescope (EHT)  is an international project aimed at taking the first picture of a black hole, specifically of Sagittarius A*, the site of the black hole that is believed to be lurking at the center of our Milky Way galaxy…..

….. Getting there will require a blend of new technology, old tricks, and the anointing of a brand-new radio telescope array that will come online over the next few years.
But Doeleman and the various collaborators building the EHT are now confident that what wasn’t even thinkable just a few years ago is now within reach, as technology has turned a time-tested astronomical technique into a tool that should give us our first glimpse of Einstein’s vision of gravity’s most violent manifestation.
That technique is very long baseline interferometry (VLBI), and it’s what allows the EHT team to build a telescope the size of Earth without actually building anything at all.
By feeding data from radio telescopes around the world into a supercomputer, they can create a telescope with an imaging area the size of the entire planet, allowing them to capture images in radio wavelengths at resolutions that should let them see straight to the heart of the Milky Way
Think of VLBI like this: You’re standing at the center of the galaxy, looking at Earth, which is way out at the Milky Way’s fringe.
Now, imagine Earth as a mirror, but only the places where there are radio telescope arrays on its surface are polished–the rest of this planet-sized mirror is blacked out.
These polished spots are the only places on the mirror that can collect data. This sparse mirror wouldn’t provide a very complete picture to someone peering through the other end.
But now imagine the Earth rotating. The polished portions of the lens–the parts collecting data–begin to slowly move across the blacked-out areas of the mirror, collecting data from different points on its surface as rotation and the seasonal tilt of the planet continue.
Eventually, the telescopes–and there are many scattered all over the globe–have collected data from positions all over this lens, just not all at the same time. Over months and years, this data is sufficient to stitch together a rather thorough view roughly equivalent to that captured by an Earth-sized telescope mirror.
That’s VLBI. By linking the data from many telescopes together, the EHT can generate a virtual telescope, with a data-gathering surface the size of the planet.
Their data is time-stamped by a hydrogen maser atomic clock, ensuring that given enough computing power, all the radio data can be neatly stitched together into a single picture. And given enough time, and as more radio telescopes come online, that picture grows clearer and clearer.
Up to a point, at least. VLBI has been employed by astronomers for decades, but an undertaking like the EHT wasn’t possible previously. The technology simply wasn’t there yet. It’s really not even there now, but it’s so close that Doeleman and his EHT colleagues can begin gathering data.
“We have the opportunity to make measurements that weren’t possible five years ago,” Doeleman says. “In the last five years, we’ve developed instrumentation to carry out VLBI at the highest frequencies where you get very good resolution.
We can also now swallow large swaths of bandwidth. Instead of a couple of hundred megahertz, we can now swallow many gigahertz.
You can think of that as being more energy, more photons from the black hole itself. That means our sensitivity goes way up. So it’s a combination of higher sensitivity and more telescopes around the Earth that’s letting us do what we couldn’t five years ago.
The technology is at a point now that it’s a matter of implementation rather than building new systems.”…..

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