How does the basic interferometry very long?

The Horizon Event Telescope creates the image of a black-hole shadow through the precise coordination of a global network of telescopes. Here's how they do it.

On April 10, scientists delighted the world by unveiling an image of the black hole in the center of the far-off galaxy M87. The shadow is enveloped by the light of the hot gas just outside the black hole, which swirls around the invisible beauty.

To create this image, an international team combined radio telescope observations from Hawaii to Spain and from Arizona to Chile. The dishes observed simultaneously act effectively as a virtual dish the size of a planet, which gives them a power of resolution equivalent to being able to see a hydrogen atom at arm's length.

But how do you build a virtual telescope?

The answer is a technique called very long basic interferometry. VLBI is actually not new; radio astronomers have been using it for decades. But no one had previously reached VLBI with a frequency that could invade the deepest sanctuary of a galaxy in such a breathtaking way.

The basic principle of interferometry is as follows: take two telescopes distant from each other and observe an object simultaneously with both telescopes. The light comes from the object in the form of a wavefront, like ripples in a pond created by splashing ducks. Both telescopes will capture a slightly different part of each wavefront. Consider this delay, then carefully add the data, and you can measure the structure of the object with the resolution obtained with a telescope the size of the distance between the two antennas.

But when you observe something with a structure at different scales, things get complicated. It's like a flock of ducks frolicking in the pond, their waves interacting and changing the pattern in a complex way. To reconstruct the image, you need to understand in detail how the radio waves increase or cancel when they go to the dishes. The solution consists of a set of telescopes with different separations, which allows you to mix and match pairs and detect structures of different sizes and orientations. That's why the Event Horizon telescope is not just two spans at opposite ends of the world: they need a variety of separations or baselines, in order to fill the image.

The animations below by graduate student Daniel Palumbo (Harvard) show how the VLBI network of the Horizon Telescope Event works. On the left, the Earth seen through the black hole of M87. As the world turns, different telescopes appear. Each forms a pair with the others, their baselines drawn in red.

Now, look at the right panel. This graph represents the distance between the telescopes divided by the observed wavelength. The x-axis is east-west; the y axis is north-south. Further from zero corresponds a larger separation and thus a finer resolution. The zero point in the center is the largest scale, what you see with a single telescope.

Each pair of dishes represents two points on this graph, because you can "read" their separation in two different ways: for example, Chile in Spain or Spain in Chile. As the Earth rotates, the position of the two sites in relation to each other, as seen by the black hole changes: the telescope in Mexico, for example, travels more distance from left to right compared to that of Chile. This rotation changes both the east-west / north-south length and the orientation of the baselines, which is why points plot arcs in the graph over time.

How much of this swirling graph, called the UV Plan, is provided indicates to astronomers the range of scales that their VLBI network can detect. If the UV plan was completely filled with red, we would have a complete picture. However, it would take a lot more telescopes than we have – the EHT satellite basically used all radio telescopes on the planet capable of observing at 1.3mm (230GHz), and there is still a lot of white space .

Instead, scientists use sophisticated computer algorithms to fill in the blanks and reconstruct the image. To do this, the team has installed atomic clocks on each telescope, devices so reliable that they will lose a second in 10 million years. These clocks mark the observations with exact timestamps. The researchers then recover the hard disks containing this time stamped data to their supercomputers and combine them. correlative, the observations of many sites, aligning them with billions of seconds. Only then can they begin to reconstruct the shadow of the black hole.

Here are the same terrestrial and UV plane panels, this time joined by the image of the shadow of the black hole they reveal. As the Earth rotates, more data at different scales are added and the sharpness of the image changes.

For the analogy with the full duck for VLBI, check out this presentation by Michael Johnson (Center for Astrophysics, Harvard & Smithsonian). If you're curious about how image reconstruction works, Katie Bouman (also CfA) did a nice TED talk explaining one of the methods of the algorithm.

Note in the animations above that the South Pole Telescope is out of range. The EHT team uses it to observe the central dark hole of the Milky Way, Sagittarius A *, and also uses it to detect a calibrator source for M87 observations. For the VLBI network seen by Sgr A *, check out this simulation by Laura Vertatschitsch (now System and Technology Research). Some of the featured sites are different from those involved in the April 2017 campaign, but the concept is the same.