Scientist to explore the Eye of Sauron

[BeBMan]

Ok, not exactly, but something interesting nonetheless:

Scientists believe they are on the verge of obtaining the first ever picture of a black hole.

They have built an Earth-sized "virtual telescope" by linking a large array of radio receivers - from the South Pole, to Hawaii, to the Americas and Europe.

There is optimism that observations to be conducted during 5-14 April could finally deliver the long-sought prize.

In the sights of the so-called "Event Horizon Telescope" will be the monster black hole at the centre of our galaxy.

Although never seen directly, this object, catalogued as Sagittarius A*, has been determined to exist from the way it influences the orbits of nearby stars.

These race around a point in space at many thousands of km per second, suggesting the hole likely has a mass of about four million times that of the Sun.

But as colossal as that sounds, the "edge" of the black hole - the horizon inside which an immense gravity field traps all light - may be no more than 20 million km or so across.

And at a distance of 26,000 light-years from Earth, this makes Sagittarius A* a tiny pinprick on the sky.

The Event Horizon Telescope (EHT) team is nonetheless bullish. (...)

Fulll article: http://www.bbc.com/news/science-environment-38937141

[SlowwHand]

I await breathlessly.

[-Jrabbit]

So we will get to see what it looked like 26,000 years ago?

That's, like, way before God created the heavens and the earth...

[Lorizael]

This is an incredibly ambitious project. Our line of sight to the center of the galaxy is blocked in many wavelengths by dust and densely populated, bright stars. The only way to see through it all is to pick a wavelength (~1.3 mm) for which the intervening stuff is mostly transparent. But the larger the wavelength of light you're looking at (1.3 mm is huge compared to visible light's 400-700 nm, for example), the larger a telescope you need to resolve images. So while something the size of an eye is good at resolving medium-sized objects here on Earth, you need something much bigger--essentially the size of the planet--to resolve an image of an object that's only a few million km across and 26,000 light-years away.

We can't build a telescope that big yet, so instead we use the incredibly clever technique of very-long-baseline interferometry. By placing multiple radio telescopes at different points across the globe, you can pretend you have one, big telescope that's the size of the distance between your telescopes. To get as detailed a map of the source as possible, you need multiple baseline lengths, which is helped in part by having a very large array of antennae, and also in part by the rotation of the Earth, which changes the projected distance between antennae relative to the source.

Edit: There's also a (critical) bit about Fourier transforms that I'm too lazy to go into right now.

[Lorizael]

In case anyone is interested, a short bit about why interferometers give you a giant virtual telescope:

If you have two radio antennae some distance apart and they're both receiving the same signal from space, then depending on the angle at which that signal is hitting the surface of the Earth, the signal may reach one antenna before it hits the other. This means the two received signals will be out of phase with each other. If you combine the signals from both antennae (with a thing called a correlator that involves precise timekeeping and some other tools), then they will interfere with each other constructively or destructively depending on the phase difference. Signals that are directly overhead are in phase and interfere constructively, but as you slide to other angles your combined signal gets weaker. Thus, because we are very good at timekeeping, the interferometer is an extremely sensitive detector of the exact angle on the sky the signal originates from and consequently gives you very good angular resolution, which is what a large telescope does.

To resolve actual images, though, you need more than something that can detect point sources. So another way of thinking about an astronomical interferometer is as a tool that measure the strength of "spatial frequencies." Because the interferometer is measuring how out of phase two signals are, a signal can be so out of phase that it comes back around to in phase again. How long that takes depends on the distance between your two antennae. So that baseline distance determines the "spatial frequency" you are sampling: how strong are the signals that repeat every X degrees on the sky.

If you have a whole bunch of antennae with different baselines and you also make use of the Earth's rotation to change your projected baseline, then you can create a mostly complete map of the spacial frequencies that compose some source. The next step is to take (something like) the Fourier transform of that map. A Fourier transform is a way of decomposing the parts that make up a signal and vice versa. (If that sounds a little hand-wavey, I wrote a post about Fourier analysis two years ago that goes into a little more detail.) So you take the transform of your spatial frequency map and out pops a map of the strength of your signal at each angle on the sky, which is all an image in astronomy ever is. (This all gets more complicated because your source is also "convolved" with the shape of your antennae, and you can never get complete spatial frequency coverage, so the image you make is always an incomplete reconstruction that you have to guess at with some smart algorithms and some knowledge of the physics of your source.)